3D Pictures

3D Pictures can create perception of 3D on 2D screen or sidewalk. There can be, for instance, the following types:

Flickering Stereo Images - 1 object taken from 2 angles and the 2 photos alternate quickly
Stereograms - chaos at first sight, but after focusing behind it, 3D image will emerge
3D Pavement Drawings - anamorphic illusions having special distortion that create an impression of 3 dimensions when seen from one particular viewpoint
Other Anamorphic Illusions - at first sight random lines painted on walls that make a complete picture only when viewed from 1 place

Flickering Stereo Images

Plunge

You almost want to jump in the water :-) (by Jim Gasperini)

Stone Gate

Virtual walk in the nature (by Jim Gasperini).

Stereograms

Butterfly Stereogram

You won't need any 3D glasses to find 3 dimensional objects in the following pictures. The trick is not to focus on the picture/screen but to FOCUS BEHIND it, as if you wanted to look through the screen. Relax, as if you were day dreaming. It might take some practice.
Another way works for me personally better. Get really close to the screen (literally, touch the screen with your nose :-), you won't see anything sharp and that's good. Now slowly move away from the screen (remember to keep the focus BEHIND the screen) and usually around 10 inches away from the screen you should see the 3D image.
You should see a green butterfly if you relax and focus through the screen. Take your time. Click the image to enlarge it so that nothing distracts you.