> 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.
What about compressing the same image with two different encoders? Or with different zlib settings? Even if the diff after lossless decompression is small, you have to somehow reconstruct the decisions that the compressor made on the target file.
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.