You can't attach PDF files as images, but I already know what you are talking about. Could you convert them to JPEG for the rest of the class?
These show up on my Mac Firefox, but not my Windows Firefox. They are very annoying, and sometimes obscure the post text. This is the only site that I use Safari for when Mac surfing.
The Mac version is still a bit behind the others, but Ben Goodger still intends to get the Mac version working well. It is just on a lower priority. See the
Mozilla Firefox 1.0 Roadmap for more details.
It is also possible that the upcoming upgrade to vBulletin 3.x will make the problem go away. I'm hoping...