Spooky image of a creature with a diabolical grin has been accidentally discovered on Aphex Twin's Windowlicker EP, a 1999 hit.
Just like the backward messages on vinyl LPs, the face is secretly encoded in the actual sound waves of the music. It isn't a separate file on the CD. The image is the aural equivalent of steganography: the practice of hiding secret messages and watermarks in images.
The existence of the image has not been publicized by Aphex Twin or his record label.
It seems to have been discovered accidentally by an electronic musician who calls himself Chaos Machine. Chaos Machine wasn't available for an interview, but according to his website, he was playing around with WinAmp one evening when he spotted the diabolical face.
His discovery can be reproduced with a sample of the song and some spectrographic software, which displays the different frequencies of the song as a graph against time. The image doesn't appear in an MP3 file of the song; the compression algorithm destroys the image.