I used to own a computer store that custom-made these kinds of images for people. They were extremely popular. The trick to viewing it is as follows:
Sit or stand roughly arm's length from the screen. Imagine you are focusing THROUGH the screen to a place roughly a foot behind it. If your primary focus is "behind" the image, your peripheral vision will pick up what the image shows. If you focus ON the image, it won't work.
Also note: If you have astigmatism, it won't work for you.
Have fun!
Dave (who has astigmatism, and has never been able to see one of these things.
)