How are these images stored, please? Is the intention to compare two or more files that might or might not be identical? Or is the idea simply to try to locate any regions of two JPEGs that look different - for example a couple of stills from a CCTV camera which would look very similar most of the time, except say when one of them shows someone walking across the shot? I've no idea how one might tackle a problem like this, but it sounds an interesting one - so can you supply more details?