If you look intensely at these black bars, you will just see black bars. But if you shake your head, you will see an image. See? No. Shake it again. Yes? OK. Have fun until you get a headache.
Now I can recommend you shake the window instead. Same effect. Also, if you relax your eyes and lose focus, you will also see the hidden image appear, just to disappear the second you refocus.
MAGIC! [Thanks Karl!]