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.
PBM is probably older than GIF and still has software support
https://en.wikipedia.org/wiki/Netpbm#File_formats
though apparently not directly in browsers! (I would have expected it did, but I couldn't get browsers to render PBM files directly.)