A bright coloured fish
MP3 files can be ātaggedā 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 āAttached picturesā that can be added to an audio file.
Among options like $10 āMovie/video screen captureā, and $03 āCover (front)ā, there is also $11 āA bright coloured fishā.