How to embed images of different aspect ratios?

[endolith]

If I'm embedding images that are not square, do I need to do cropping or padding first to make them square? Which is better? Were the models trained on different aspect ratios? Did they use cropping or padding? If padding, should it be white or black or doesn't matter? I'd prefer padding, because I want to preserve all the original image and estimate image composition, etc. but not if it decreases accuracy.