David at Ironic Sans had a cool idea: hide an image in the histogram of another image. Not a generally useful technique, but a neat hack that demonstrates that a stream of data may be interpretable in unusual ways. Nick Fisher took it one step further and made an online tool for creating these.
A commenter points to images hidden in audio spectrographs, a similar idea.
All these techniques are forms of steganography, the art of hiding messages inside other messages. It's a form of secret communication complete distinct from cryptography, and has a long, rich and fascinating history.
Completely off-topic: Ironic Sans also has a witty line of pre-pixelated T-shirts for reality TV.