You're likely thinking of SID music files. SID was the Audio chip in the C64. Damn good sound for the time. There was a large community of amateur composers and cover artists producing songs in the SID-Player format. But the awe-ispiring music came from some very talented composers really showing off the machine with more advanced music software/programming and later track editors: Rob Hubbard, Chris Hülsbeck, Ben Daglish, David Whittaker and a bunch of others. I still remember the first time I heard a sampled sound on my C64. The first time I sampled a sound myself, and the first time I ever heard a composed piece using sampled instruments.

Nostalgia. Then of course came the Amiga which took a very long time to be bested...

And to get things back on topic (sort-of) the Amiga's most popular image format was IFF-ILBM (Interchange File Format - Interleaved Bitmap). And IFF was passed around as a format idea when PNG was first being developed - not adopted.

Bruno