Stereograms
Single-image stereograms, or autostereograms, are images that when viewed correctly gives the perception of 3D depth. Viewing them requires some training, as our eyes are not used to converging/diverging our focus. Tools can therefore be helpful for those who have difficulties seeing the 3D shapes.
See also: Code-Breaking overview | Exif | Low contrast images | Music steganography | Openpuff | Outguess | Spectrograms | Steghide