Actually, Kim managed to make the BMP files into WAV files and the mod still worked! Images included, despite being WAV. That was in 2015.
Cave Editor can convert the files, as well as remove the whole (c) Pixel requirement that the PBM files have.
Of course, I don't know why people go for BMP usually. (ZarroTsu's rendition of Jenka's Nightmare has JNB files instead.)