I would suggest that editing the image and saving it (without actually making changes) is actually altering the compression level of the jpg So, in fact, the image has changed.

If it's not actually changing the compression level, then I would guess the jpg header is being modified. Compare the files with a hex editor and then, if that is the case, strip out the header string prior to comparison.

In the end, it sounds like you want more of a soundex() for images? Can't help there. Sorry.