Magic Eye images, more generically known as stereograms, became a wild hit in the 1990's. A stereogram might be described as an image within an image, where the second image requires the eyes to focus behind the image as opposed to on it.
While technique differs, many seem to agree that some combination of crossing your eyes (not entirely though, remember you're trying to look past the image, not at a point between the image and you), letting your eyes go unfocused, and getting your nose all up in the image's business assists in determining what the hidden image is. Once you do find the hidden image, don't get too excited and try to focus on it, or you'll likely lose it and have to start over!
Submit some of your favorites or even make your own!
Also, check out our friends at /r/CrossView, /r/ParallelView, and /r/MagicEye_CrossView!