There are many ways to create an optical illusion. Digitally manipulating a photograph using software such as Photoshop or Gimp creates convincing images, but like special effects in the movies you know they are fake.
There are ways to setup true photographic optical illusions without altering or "faking" the image. Take the following actual photo as an example.
This modern photograph is reminiscent of Sandro Del Prete's famous illusion drawing "Impossible Chess Set" below.
Impossible Chess Set
Sandro Del Prete
So how do you think the photo "Chess Anyone?" was accomplished?
One key to photographic illusions is the fact that photos are 2 dimensional images of 3 dimensional objects. This means most photo illusions would fall apart if you saw them first hand. If you wish to see the Chess Anyone illusion from an angle that will reveal how it was accomplished place your mouse over the below reveal screen.
Chess Anyone? Revealed
For a complete tutorial on how this photographic optical illusion was done read Tim Jensen's explanation here.