Those visual cryptos are quite fun. You can even make a third image out of two other ones. I'm still trying to figure out if you can do this in multiple steps though, like sheet A+B, B+C, A+C are all different images, and A+B+C is yet a different one again (and preferably A,B,C, would be images themselves, but it's probably easier if you let them to be 'noise'). ...
