efficient way to compare 2 images?

[madscijr]

How do you want to compare these two pictures to find out the difference?

That's what I'm wondering. 
Maybe there's some magic "compare object" command that I don't know about. 
Evidently comparing it pixel-by-pixel, or a representative sampling of pixels, is a way to do it. 
But see my reply to Pete above to see why I'm asking this, there might be a better approach than comparing images...