(Screenshot by CBSi)
Here's a novel video that shows you how to use your camera's manual controls to uncover hidden images.
Grab any camera with manual controls, like an SLR, and watch the video below. You'll see a set of instructions on what to set your exposure to, and prompts for when to take photos.
The moving pattern of pixels form an image that can only be seen when captured in a long exposure. Very neat.
If you don't have a camera on hand, then you can watch this video to see what the hidden images are all about.