Comment by colechristensen

Any image manipulation program like photoshop with layers, you put the suspect images on top of one another and use filters to subtract one layer from the other (I'm not sure which filter operation works best, it might be multiply or divide) and then work to align the two layers. Differences and similarities become extremely obvious.

You can also get the raw pixel information by converting to a bitmap and comparing values, but it's easier visually because it's pretty trivial for a simple image modification to change all of the pixel values but still have the same image.