That's strange. Maybe it has to do with the recent software upgrade to the forums, because I don't think there are any restrictions on avatar usage. Perhaps try posting in the Xenforo thread or messaging one of the admins.
http://www.cavestory.org/forums/threads/quickie-guide-to-the-mods-section-update-7-04-14.4164/ This is probably the easiest way, but it hasn't been updated for a while, so it's not a recent list. If that doesn't work for you, you can try a Google advanced search.