I've played around with this idea in the past. Here is a demo with three images encrypted in two different images.

use the arrow keys to move the images around or drag them.

If you overlap them perfectly, you see one image. 5 pixels left of that is another image, and 5 pixels right of center is a third image. They were originally photos and end up very high contrast :-)

http://bettermagician.com/topsecret/

Here is a version with only two images though with a better grayscale. due to the method I was trying you can see some information leak through: http://bettermagician.com/topsecret/big.html