Everybody knows human have binocular eyesight. Left and right eyes sees different images of object - from different visual angle. Human brain are computing object distance, relying on this difference.
Pair of photos (left photo for right eye, right for left) may cause illusion of real space in the flat image. You should cut eyes so that between images the third (central) image has appeared. Focus on this virtual image.
There is more methods, like using red and green color filters and merging image in one, using repeating textures with displacement of parts of texture, or using animated picture. Images of that types are wellcome too.