At least some (possibly many) of these predate PNG, which was introduced in 1996 and didn’t really become ubiquitous until the 2000s. Besides, writing a PNG decoder would have been a huge task, and with demoscene’s strong DYI culture, relying on external dependencies was not a big thing (oh how far we have come…) Not that there even were many third-party libraries readily available, in a time before GitHub or SourceForge or even the web itself!
The source format stored with the assembly source code was often a .raw file. I guess it's an uncompressed file with per pixel color values that was included directly in the asm.
