Some of the best stereograms (A hidden 3D image in a 2D image)
If you are not able to see, here are some help:
http://www-ai.ijs.si/sirds/SirdsFaq.html#Subject3
http://iyi.yi.org/junoi/grafix/stereograms/theory/how2c.html
http://www.eyetricks.com/stereograms/stereowhy.htm
And here is Training Gallery:
http://www.aolej.com/stereo/training.html 

Could you view all of 3D images in these Stereograms?