01. Convert a picture to an audio file with Photosounder, MetaSynth or Sonic Photo (for Windows only)
02. Save the audio file (not possible with free demo of Photosounder and MetaSynth, but you can record the sound with Audacity and save it)
03. Use the spectrogramm (programme: Sonic Visualiser) to make the picture, hidden in the generated audio file, visible
It has been published in “Fake it! Fake them! Fake you! Fake us!” a book by Hackers & Designers.