This is one of those issues where I have an opinion but I honestly don't care that much either way. If the place I worked had one method in place I'd probably d

This is one of those issues where I have an opinion but I honestly don't care that much either way. If the place I worked had one method in place I'd probably do that. The nice thing about doing it on upload is It allows you to offload the processing to a job queue so you don't take a huge performance hit. (This is what I've done in the past). If you're resizing to sizes not relative to the original it also gives you the ability to maybe do some editing/cropping to make sure the resized images still make sense. Digg has a tool like this for avatar images.