Tool for comparing two images?

Beyond Compare is really a file-and-folder comparison tool, but it also contains image comparisons (example link).

There's a free trial. I bought this tool ($30) because it's so incredibly useful for many things.

I use AKS

It allows you to compare two JPG files and show differences between them. For instance, this tool allows you to compare two versions of the same image, chart, diagram or scheme.

Okay this is going to sound ridiculous, but if they are the same size you can put them side by side and cross your eyes like you would for a magic eye 3D picture, and the differences will stand out as if they are flickering (as your brain quickly alternates between the two different overlapped picture elements)

I use this "cheat" when I play those spot-the-difference flash games :P