Beyond Compare by Scooter Software is probably best for finding differences in pictures down to the single pixel level. It's not open source, but it is free for a month or so.

You can browse in a folder tree view, choose which pictures to compare, and it brings them up side by side, with a gray over laid third image, showing differences in color.

If you are looking for similar images through recursive directories, Geeqie does a good job, even finding cropped and rotated variations. Geeqie's interface isn't the best, but it seems to the best at finding duplicates. . like in my wife's phone photo collection.