I think these work because the human brain will try to find patterns in visual objects. These stereograms have so many possible combinations, and our brains switch between the different "organizations" very rapidly because there is no stable combination or pattern. These images with pictures in them are made so that our brains will eventually organize them into a pattern that looks like a picture. If the human brain tries to organize something with multiple colors or that is moving, it gets confused and tries to organize it so fast that it "glitches" and affects other areas of the brain, sometimes profoundly. Click
here to see what it feels like when your brain tries to organize something that is nearly impossible to organize
(WARNING!).