Reversed stereogram of Lincoln
To see the stereo effect, look at the large version of the image, stare right between the two figures with your head straight, and cross your eyes until you see four Lincoln figures. Now slowly uncross your eyes (but not completely) until you see three figures. The middle one is the convergence of the two inner figures, and that's the one on which you concentrate. After a while, your eyes should naturally bring that middle figure into focus while remaining crossed. Then you can "look around" the 3D image a bit, but wander off too far or tilt your head too much, and the effect goes away.