diff --git a/checkstyle-filter.xml b/checkstyle-filter.xml
index fd0a250..d4366c4 100644
--- a/checkstyle-filter.xml
+++ b/checkstyle-filter.xml
@@ -12,8 +12,9 @@
   <!-- Seems unavoidable. -->
   <suppress files="LFileModel.java"
             checks="ClassDataAbstractionCoupling"/>
-  <!-- Seems unavoidable. -->
   <suppress files="LFileModel.java"
             checks="MethodCount"/>
+  <suppress files="LFileModel.java"
+            checks="FileLength"/>
 
 </suppressions>
diff --git a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/LFileModelType.java b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/LFileModelType.java
index 991b5e4..a564544 100644
--- a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/LFileModelType.java
+++ b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/LFileModelType.java
@@ -554,4 +554,18 @@ CompletableFuture<?> captionsPaste(
    */
 
   void exportClear();
+
+  /**
+   * Change the source of an image.
+   *
+   * @param image  The image
+   * @param source The source
+   *
+   * @return The operation in progress
+   */
+
+  CompletableFuture<?> imageSourceSet(
+    LImageID image,
+    URI source
+  );
 }
diff --git a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LCommandImageSourceSet.java b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LCommandImageSourceSet.java
new file mode 100644
index 0000000..fb3a996
--- /dev/null
+++ b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LCommandImageSourceSet.java
@@ -0,0 +1,164 @@
+/*
+ * Copyright © 2024 Mark Raynsford <code@io7m.com> https://www.io7m.com
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+ * IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+
+package com.io7m.laurel.filemodel.internal;
+
+import com.io7m.laurel.model.LException;
+import org.jooq.DSLContext;
+
+import java.util.Properties;
+
+import static com.io7m.laurel.filemodel.internal.Tables.IMAGES;
+
+/**
+ * Set an image source.
+ */
+
+public final class LCommandImageSourceSet
+  extends LCommandAbstract<LImageSourceSet>
+{
+  private SavedData savedData;
+
+  private record SavedData(
+    long savedImageId,
+    String savedOldURI,
+    String savedNewURI)
+  {
+
+  }
+
+  /**
+   * Set an image source.
+   */
+
+  public LCommandImageSourceSet()
+  {
+
+  }
+
+  /**
+   * @return A command factory
+   */
+
+  public static LCommandFactoryType<LImageSourceSet> provider()
+  {
+    return new LCommandFactory<>(
+      LCommandImageSourceSet.class.getCanonicalName(),
+      LCommandImageSourceSet::fromProperties
+    );
+  }
+
+  private static LCommandImageSourceSet fromProperties(
+    final Properties p)
+  {
+    final var c = new LCommandImageSourceSet();
+
+    c.savedData = new SavedData(
+      Long.parseUnsignedLong(p.getProperty("image")),
+      p.getProperty("oldURI"),
+      p.getProperty("newURI")
+    );
+
+    c.setExecuted(true);
+    return c;
+  }
+
+  @Override
+  protected LCommandUndoable onExecute(
+    final LFileModel model,
+    final LDatabaseTransactionType transaction,
+    final LImageSourceSet request)
+    throws LException
+  {
+    final var context =
+      transaction.get(DSLContext.class);
+
+    final var old =
+      context.select(IMAGES.IMAGE_SOURCE)
+        .from(IMAGES)
+        .where(IMAGES.IMAGE_ID.eq(request.image().value()))
+        .fetchOne(IMAGES.IMAGE_SOURCE);
+
+    final var updated =
+      context.update(IMAGES)
+        .set(IMAGES.IMAGE_SOURCE, request.source().toString())
+        .where(IMAGES.IMAGE_ID.eq(request.image().value()))
+        .execute();
+
+    if (updated == 0) {
+      return LCommandUndoable.COMMAND_NOT_UNDOABLE;
+    }
+
+    this.savedData = new SavedData(
+      request.image().value(),
+      old,
+      request.source().toString()
+    );
+
+    model.setImagesAll(LCommandModelUpdates.listImages(context));
+    return LCommandUndoable.COMMAND_UNDOABLE;
+  }
+
+
+  @Override
+  protected void onUndo(
+    final LFileModel model,
+    final LDatabaseTransactionType transaction)
+  {
+    final var context =
+      transaction.get(DSLContext.class);
+
+    context.update(IMAGES)
+      .set(IMAGES.IMAGE_SOURCE, this.savedData.savedOldURI)
+      .where(IMAGES.IMAGE_ID.eq(this.savedData.savedImageId))
+      .execute();
+
+    model.setImagesAll(LCommandModelUpdates.listImages(context));
+  }
+
+  @Override
+  protected void onRedo(
+    final LFileModel model,
+    final LDatabaseTransactionType transaction)
+  {
+    final var context =
+      transaction.get(DSLContext.class);
+
+    context.update(IMAGES)
+      .set(IMAGES.IMAGE_SOURCE, this.savedData.savedNewURI)
+      .where(IMAGES.IMAGE_ID.eq(this.savedData.savedImageId))
+      .execute();
+
+    model.setImagesAll(LCommandModelUpdates.listImages(context));
+  }
+
+  @Override
+  public Properties toProperties()
+  {
+    final var p = new Properties();
+    p.setProperty("image", Long.toUnsignedString(this.savedData.savedImageId));
+    p.setProperty("oldURI", this.savedData.savedOldURI);
+    p.setProperty("newURI", this.savedData.savedNewURI);
+    return p;
+  }
+
+  @Override
+  public String describe()
+  {
+    return "Set image source";
+  }
+}
diff --git a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LFileModel.java b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LFileModel.java
index 6e3d87b..5b5c7dd 100644
--- a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LFileModel.java
+++ b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LFileModel.java
@@ -1225,6 +1225,20 @@ public void exportClear()
     this.exportEvents.set(List.of());
   }
 
+  @Override
+  public CompletableFuture<?> imageSourceSet(
+    final LImageID image,
+    final URI source)
+  {
+    Objects.requireNonNull(image, "image");
+    Objects.requireNonNull(source, "source");
+
+    return this.runCommand(
+      new LCommandImageSourceSet(),
+      new LImageSourceSet(image, source)
+    );
+  }
+
   private Optional<InputStream> executeImageStream(
     final LImageID id)
     throws LException
diff --git a/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LImageSourceSet.java b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LImageSourceSet.java
new file mode 100644
index 0000000..c994dcf
--- /dev/null
+++ b/com.io7m.laurel.filemodel/src/main/java/com/io7m/laurel/filemodel/internal/LImageSourceSet.java
@@ -0,0 +1,48 @@
+/*
+ * Copyright © 2024 Mark Raynsford <code@io7m.com> https://www.io7m.com
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR
+ * IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+
+package com.io7m.laurel.filemodel.internal;
+
+import com.io7m.laurel.model.LImageID;
+
+import java.net.URI;
+import java.util.Objects;
+
+/**
+ * A request to set a source for an image.
+ *
+ * @param image  The image
+ * @param source The source
+ */
+
+public record LImageSourceSet(
+  LImageID image,
+  URI source)
+{
+  /**
+   * A request to set a source for an image.
+   *
+   * @param image  The image
+   * @param source The source
+   */
+
+  public LImageSourceSet
+  {
+    Objects.requireNonNull(image, "image");
+    Objects.requireNonNull(source, "source");
+  }
+}
diff --git a/com.io7m.laurel.filemodel/src/main/java/module-info.java b/com.io7m.laurel.filemodel/src/main/java/module-info.java
index 82fe64d..aa17563 100644
--- a/com.io7m.laurel.filemodel/src/main/java/module-info.java
+++ b/com.io7m.laurel.filemodel/src/main/java/module-info.java
@@ -31,6 +31,7 @@
 import com.io7m.laurel.filemodel.internal.LCommandImageCaptionsAssign;
 import com.io7m.laurel.filemodel.internal.LCommandImageCaptionsUnassign;
 import com.io7m.laurel.filemodel.internal.LCommandImageSelect;
+import com.io7m.laurel.filemodel.internal.LCommandImageSourceSet;
 import com.io7m.laurel.filemodel.internal.LCommandImagesAdd;
 import com.io7m.laurel.filemodel.internal.LCommandImagesDelete;
 import com.io7m.laurel.filemodel.internal.LCommandMetadataPut;
@@ -85,6 +86,7 @@
     LCommandImageCaptionsAssign,
     LCommandImageCaptionsUnassign,
     LCommandImageSelect,
+    LCommandImageSourceSet,
     LCommandImagesAdd,
     LCommandImagesDelete,
     LCommandMetadataPut,
diff --git a/com.io7m.laurel.gui/src/main/java/com/io7m/laurel/gui/internal/LCaptionsView.java b/com.io7m.laurel.gui/src/main/java/com/io7m/laurel/gui/internal/LCaptionsView.java
index 0ffb47e..3ce9c09 100644
--- a/com.io7m.laurel.gui/src/main/java/com/io7m/laurel/gui/internal/LCaptionsView.java
+++ b/com.io7m.laurel.gui/src/main/java/com/io7m/laurel/gui/internal/LCaptionsView.java
@@ -44,6 +44,7 @@
 import javafx.scene.control.TableColumn;
 import javafx.scene.control.TableView;
 import javafx.scene.control.TextField;
+import javafx.scene.control.TextInputDialog;
 import javafx.scene.image.ImageView;
 import javafx.scene.layout.Pane;
 import javafx.stage.Stage;
@@ -52,6 +53,7 @@
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
+import java.net.URI;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -59,6 +61,7 @@
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import static com.io7m.laurel.gui.internal.LStringConstants.IMAGES_SOURCE_SET;
 import static javafx.stage.Modality.APPLICATION_MODAL;
 
 /**
@@ -96,6 +99,8 @@ public final class LCaptionsView extends LAbstractViewWithModel
   @FXML private MenuItem assignedCaptionsContextMenuCopy;
   @FXML private MenuItem assignedCaptionsContextMenuPaste;
   @FXML private MenuItem imagesCompareCaptions;
+  @FXML private TextField imageSource;
+  @FXML private Button imageSourceButton;
 
   private Stage imageDisplayWindow;
   private LImageView imageDisplay;
@@ -145,6 +150,7 @@ protected void onInitialize()
     this.imageDelete.setDisable(true);
     this.imageCaptionAssign.setDisable(true);
     this.imageCaptionUnassign.setDisable(true);
+    this.imageSourceButton.setDisable(true);
     this.captionNew.setDisable(false);
     this.captionDelete.setDisable(true);
     this.captionModify.setDisable(true);
@@ -413,10 +419,19 @@ private void onImageSelected(
       fileModel.imageSelect(Optional.empty());
       this.imageView.setImage(null);
       this.imageDelete.setDisable(true);
+      this.imageSourceButton.setDisable(true);
+      this.imageSource.setText("");
       return;
     }
 
     this.imageDelete.setDisable(false);
+    this.imageSourceButton.setDisable(false);
+    this.imageSource.setText(
+      image.image()
+        .source()
+        .map(URI::toString)
+        .orElse("")
+    );
 
     fileModel.imageSelect(Optional.of(image.id()));
     fileModel.imageStream(image.id())
@@ -723,4 +738,24 @@ private void onCaptionsCompareSelected()
 
     this.comparisons.open(this.services, this.fileModelScope());
   }
+
+  @FXML
+  private void onImageSetSourceSelected()
+  {
+    final var dialog = new TextInputDialog();
+    LCSS.setCSS(dialog.getDialogPane());
+
+    dialog.getEditor().setText(this.imageSource.getText());
+    dialog.setHeaderText(this.strings.format(IMAGES_SOURCE_SET));
+
+    final var r = dialog.showAndWait();
+    if (r.isPresent()) {
+      final var text = URI.create(r.get());
+      final var fileModel = this.fileModelNow();
+      fileModel.imageSourceSet(
+        fileModel.imageSelected().get().orElseThrow().id(),
+        text
+      );
+    }
+  }
 }
diff --git a/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/Messages.properties b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/Messages.properties
index ed34e41..5e2ab29 100644
--- a/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/Messages.properties
+++ b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/Messages.properties
@@ -32,6 +32,7 @@ captions.compare.no_extra_captions_present=No captions are present in this\nimag
 captions.compare.presenceA=The captions that are on image A but not image B.
 captions.compare.presenceB=The captions that are on image B but not image A.
 captions.compare=Compare captions
+captions.source=Image source URI
 captions.tooltip.add=Add new caption.
 captions.tooltip.add_to_image=Add the selected caption to the current image.
 captions.tooltip.delete=Delete the selected caption.
@@ -40,6 +41,7 @@ captions.tooltip.modify=Modify the selected caption.
 captions.tooltip.priority_down=Reduce caption priority.
 captions.tooltip.priority_up=Increase caption priority.
 captions.tooltip.remove_from_image=Remove the selected caption from the current image.
+captions.tooltip.source.set=Set image source URI...
 captions.unassigned=Unassigned
 categories.tooltip.add=Add a new category.
 categories.tooltip.assign=Assign captions to the category.
@@ -65,6 +67,7 @@ globals=Global prefix captions
 history.compact_confirm=Are you sure you want to delete the undo/redo history and compact the file? This operation cannot be undone.
 history.tooltip.compact=Delete history and compact database...
 history=History
+images.source.set=Set a new source URI.
 image=Image
 images.compare_captions=Compare captions...
 images.filename=Filename
@@ -100,8 +103,8 @@ title.unsaved=Laurel ({0}) *
 title=Laurel
 undo=Undo
 undo_specific=Undo ({0})
-validation=Validation
+validation.good=Dataset successfully validated without any issues.
 validation.tooltip.execute=Run validation now.
 validation.tooltip.goto=Go to the offending image.
-validation.good=Dataset successfully validated without any issues.
+validation=Validation
 
diff --git a/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/captions.fxml b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/captions.fxml
index 203c9d4..f1c507a 100644
--- a/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/captions.fxml
+++ b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/captions.fxml
@@ -31,6 +31,7 @@
           <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="32.0" vgrow="SOMETIMES" />
           <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="32.0" vgrow="SOMETIMES" />
             <RowConstraints vgrow="SOMETIMES" />
+            <RowConstraints maxHeight="-Infinity" minHeight="-Infinity" prefHeight="32.0" vgrow="SOMETIMES" />
         </rowConstraints>
          <children>
         <StackPane styleClass="gridMember">
@@ -113,6 +114,23 @@
                   </ContextMenu>
                </contextMenu>
         </TableView>
+            <HBox prefHeight="100.0" prefWidth="200.0" styleClass="gridMember" GridPane.rowIndex="4">
+               <children>
+                  <TextField fx:id="imageSource" editable="false" maxHeight="1.7976931348623157E308" maxWidth="1.7976931348623157E308" promptText="%captions.source" HBox.hgrow="ALWAYS" />
+                  <Button fx:id="imageSourceButton" disable="true" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" mnemonicParsing="false" onAction="#onImageSetSourceSelected" prefHeight="32.0" prefWidth="32.0">
+                     <tooltip>
+                        <Tooltip text="%captions.tooltip.source.set" />
+                     </tooltip>
+                     <graphic>
+                        <ImageView fitHeight="24.0" fitWidth="24.0" pickOnBounds="true" preserveRatio="true">
+                           <image>
+                              <Image url="@image-source-set.png" />
+                           </image>
+                        </ImageView>
+                     </graphic>
+                  </Button>
+               </children>
+            </HBox>
          </children>
       </GridPane>
       <GridPane styleClass="gridVisible" HBox.hgrow="ALWAYS">
diff --git a/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/image-source-set.png b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/image-source-set.png
new file mode 100644
index 0000000..1e401cf
Binary files /dev/null and b/com.io7m.laurel.gui/src/main/resources/com/io7m/laurel/gui/internal/image-source-set.png differ
diff --git a/com.io7m.laurel.tests/src/main/java/com/io7m/laurel/tests/LFileModelTest.java b/com.io7m.laurel.tests/src/main/java/com/io7m/laurel/tests/LFileModelTest.java
index 35db436..2e7b7ad 100644
--- a/com.io7m.laurel.tests/src/main/java/com/io7m/laurel/tests/LFileModelTest.java
+++ b/com.io7m.laurel.tests/src/main/java/com/io7m/laurel/tests/LFileModelTest.java
@@ -44,6 +44,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.net.URI;
 import java.nio.file.Files;
 import java.nio.file.OpenOption;
 import java.nio.file.Path;
@@ -1549,6 +1550,80 @@ public void testGlobalCaptionsModifyNonexistent()
     assertEquals(tx0, tx1);
   }
 
+  @Test
+  public void testImageSourceSet()
+    throws Exception
+  {
+    this.model.imageAdd(
+      "image-a",
+      this.imageFile,
+      Optional.of(this.imageFile.toUri())
+    ).get(TIMEOUT, SECONDS);
+
+    this.model.imageAdd(
+      "image-b",
+      this.imageFile,
+      Optional.of(this.imageFile.toUri())
+    ).get(TIMEOUT, SECONDS);
+
+    this.model.imageAdd(
+      "image-c",
+      this.imageFile,
+      Optional.of(this.imageFile.toUri())
+    ).get(TIMEOUT, SECONDS);
+
+    final var imagesThen = this.model.imageList().get();
+    final var i0 = imagesThen.get(0).id();
+    final var i1 = imagesThen.get(1).id();
+    final var i2 = imagesThen.get(2).id();
+
+    this.model.imageSourceSet(i1, URI.create("urn:example"))
+      .get(TIMEOUT, SECONDS);
+
+    assertEquals(
+      imagesThen.get(0),
+      this.model.imageList().get().get(0)
+    );
+    assertEquals(
+      URI.create("urn:example"),
+      this.model.imageList().get().get(1).image().source().get()
+    );
+    assertEquals(
+      imagesThen.get(2),
+      this.model.imageList().get().get(2)
+    );
+
+    this.model.undo().get(TIMEOUT, SECONDS);
+
+    assertEquals(
+      imagesThen.get(0),
+      this.model.imageList().get().get(0)
+    );
+    assertEquals(
+      imagesThen.get(1),
+      this.model.imageList().get().get(1)
+    );
+    assertEquals(
+      imagesThen.get(2),
+      this.model.imageList().get().get(2)
+    );
+
+    this.model.redo().get(TIMEOUT, SECONDS);
+
+    assertEquals(
+      imagesThen.get(0),
+      this.model.imageList().get().get(0)
+    );
+    assertEquals(
+      URI.create("urn:example"),
+      this.model.imageList().get().get(1).image().source().get()
+    );
+    assertEquals(
+      imagesThen.get(2),
+      this.model.imageList().get().get(2)
+    );
+  }
+
   private List<LCaptionID> globalCaptionsNow()
   {
     return this.model.globalCaptionList()