efficient way to compare 2 images?

[madscijr]

Couldn't you just:
Put image1 on a screen
Put image2 on the screen with a XOR  

Any pixels that aren't the background color will be pixels that are different in the two images.  
Count them (well, have a program do that ).  How many pixels need to be different before you can make a decision on comparison is up to you.

That's kind of brilliant, but how do you put an image with XOR??