@Olivier5,
I think you're referring to the stereogram, or what used to be called Magic Eye. It was the rage a number of years ago, even appearing in the Sunday newspaper.
It's a strange phenomenon, and I don't understand how anybody could come up with this idea. The two I posted are slightly pornographic, but it takes quite a bit of effort and concentration to see the image in 3D, so I guess the eye strain is the punishment. We'll all go blind looking at that stuff.
)
