> A bit-exact differ for, say, jpegs, probably wouldn't work all that well.

Depends. For some uses (e.g. a change in one corner of the file, encoded by the same tool with the same parameters), the diff of JPEG would be fine, differences will be restricted to the 8x8 pixel blocks that were touched by the change. Other changes (e.g., changing encoding quality, trimming a row of pixels off the edge of an image) would lead to more complex diffs.