MP3 files can be ‘tagged’ with various pieces of data. I wanted to experiment with putting bad data in an MP3 file to do all sorts of nefarious things. I found a piece of software called Kid3 which can be used to write this data. I noticed a strange option for an image field
This is actually part of the spec.
The ID3 specification defines various “Attached pictures” that can be added to an audio file.
Among options like $10 ‘Movie/video screen capture’, and $03 ‘Cover (front)’, there is also $11 ‘A bright coloured fish’.