They also have different teeth and clothes. My RAM is crap, so I can't open the image right now, but from memory, the two outside characters (not including the upside-down one or the one in the first-story "celler" (
) are similar to Meagly McGraw and Filthy Hands Floyd, while the middle one is more like Mortimar Bang or Palooka Pack Rat.
I just spotted this while typing, but compare the skull on the badge on Filthy Hands Floyd's hat and the heads of the characters in the rockart - does anyone see a similarity? They're both pretty generic, but I just saw the badge, and it reminded me of those ghostly figures.