IMG tags are enabled for Platinum members. HTML is only enabled for COB's, mods and admins for the purpose of advertising. I don't see youtube or HTML being enabled for members, but I can see admin disabling HTML for mods (*cough* like me) who use it for encoding youtube and not advertisements.
There's a link somewhere here that shows you what's enabled in the V-Bul here and what's not, including how to use multi-quote functions and stuff. I don't recall where it is offhand, though.