Most of the people here will be unaware of stereograms, so they can refer to http://en.wikipedia.org/wiki/Stereogram .
And Please dont merge this topic in optical illusions as these are very much different thing and require special technique to see the object hidden in image unlike the other illusions.
Here is the tutorial to see the stereogram images. Maybe at first you wont be able to see but constant practice will do the work.
Here's some tips how to view stereograms:
Learning how to see autostereograms in like learning how to ride a bicycle, or how to swim. Some people learn very quickly, others on the other hand experience difficulties at first, but eventually everyone manages Once you have got the hang of it, three dimensional viewing becomes increadibly easy too, no matter how long you had to practice at first. This page hopes to give you an insight into how to view autostereagrams if you can't already.
Basically you need to unfocus your vision and look through the tereogram. This makes patterns to overlap and gives each eye slightly different view, and that makes you see 3D image.
Get any still stereogram for training purposes or use the one on the bottom of game's Intro screen.
Look through the stereogram imaging that it's transparent. Your eyes should be relaxed and not focused on stereogram. When your sight is blurred try to move closer and further toward the stereogram, and do it very slowly. At some distance you may notice something happens and blurry relief may start appearing behind your screen. It'll be blurry, the next step is to make it sharp.
Try not to switch your sight onto the image - look through it all the time. Hold your head horizontally, don't tilt. Also try not to blink. It may take a little while until eyes focus properly by themselves. Do not force it because you gonna switch to 2D image. Eventually hidden image will become sharp and clear like a reflection in the mirror.
I can't see anything, what now??
If you don't manage to see the amazing pictures hidden inside a the autostereograms immediatley, don't let it get you down. Your brain needs time to switch between 'NORMAL' vision to that of stereoscopic vision. Try not to force your eyes when focusing.
Don't overdo it... Even although the methods are completely safe you should increase your viewing time gradually at first. Then, when you are sure you have mastered the technique, you can submerge yourself in the world of 3D vision for hours !
Method 1 - Parallel Method
Move your head so close to the screen that the tip of your nose nearly touches it. Stare straight ahead into the distance as though you could see right through the screen. Now, move your head slowly away from the screen. Make sure you don't change the focus of your eyes. Keep looking into the distance. As soon as you see a 3D picture is beginning to form, stay where you are. Don't change your position in front of the screen. If you should happen to loose the three dimensional picture, it is probably because you have subconciously tried to focus on the picture. If that happens, Go back to the start of the method and try again.
If you are successful then you must have seen this shark.
Good Luck and try these!!