This is sometime called Eye Magic or Illusion
This is the trickiest Eye exersise to be able to see the hidden images
A stereogram is an optical illusion of depth created from flat, two-dimensional image or images. Originally, stereogram referred to a pair of stereo images which could be viewed using stereoscope. Other types of stereograms include anaglyphs and autostereograms.
To view these Images through Naked Eyes, cross your eyes until the images appear.
(Click on the Thumbnail to Enlarge)