Visual cryptography
An example:
The original image scaled down to 240x320 pixel is first transformed into a black/white picture:
Then two random, but interconnected pictures are produced (one is the key, the other derived from it and the black/white picture, produced with no chosen pixel and pseudorandom seed #0 here):
When overlayed they show this:
Finally, key and cryptogram are embeddid in a PDF that can be easily printed on slides. Then see it work.