Netscape Navigator supported PBM (at least in some versions you could even write directly into a frame/window via JS in order to compose an image on the fly).
I completely forgot about that! I very vaguely remember experimenting with it, and I think I never got it to work. I can't find anything on it now, though.
Huh! So in that case, it could be that there's an old PBM file somewhere that could be a contender for this record.