Starting with IH5, you can choose the naming convention used for re-sized files created by the image-handler, one of Hashed or Readable.
This is the convention used by image-handler versions prior to 4.3.4. The handler uses an MD5 hash to compress the resized file's original path, name and parameters to produce file names similar to 8240eb50da20af3ecec990d3e56099fa.image.50x40.gif. It can be "very difficult" to determine which original file is associated with that resized file!.
If you are currently using an Image Handler version prior to 4.3.4, this naming-convention will be initially configured on your IH upgrade. Some stores use HTML img tags in their category and/or product descriptions that reference an image's "hashed" name and this default is set to ensure that downward compatibility.
This is the convention introduced image-handler versions later than 4.3.3. The handler concatenates the original file's path, name and parameters to produce file names similar to matrox-mg400-32mbgif.image.50x40. It's a little easier to determine which original file is associated with that resized filename!
If you are performing an initial install or upgrading Image Handler from a version later than 4.3.3, this naming will be initially configured for your IH5 installation.