I've recently saw the Seinfeld episode where Kramer purchases a 3d art picture, and Mr. Pete can't see it. Anyway, it made me get interested in it, and I looked for some of those kind of pictures in the internet - they're called Stereograms.
First I couldn't see anything, except messy curves and shapes, but then after I read the description of how to see it, it was AMAZING!
Here's an image I created with a program I found. Can anyone see the hidden image?
To see it, put your face as close to the monitor as possible, so you see the image really out of focus. Now, SLOWLY go back, while trying to keep the image blurry. At the first few times it takes a lot of time till you see the 3d image, but keep trying - it's really something when the image all of a sudden pops into the screen!
If you're succecful, there are a lot more beautiful pictures of that kind. Just google STEREOGRAMS.