Beyond Compare can open image files:
Caveat: If the images are different sizes Beyond Compare will not stretch or resize them for you. It is meant as a pixel comparison, so resizing wouldn't really make sense anyway.
Drag two images into Beyond Compare. If the filenames end in .jpg, .gif, .tiff, or similar the software will detect the file extension and open them in a Picture Compare session.
The first two controls you should start with are (1) press the Tol (tolerance) button and then play with the tolerance slider.
(2) Flip rapidly between Image A and Image B by hitting the F8 key in Windows or with your mouse: and