Due to some creative abuse of the img BBCode which was diccovered by the authors of the forum software, images are currently restricted to the following extensions: jpg, jpeg, gif, png. If the img tag doesn't end with that, and has anything else after it, it'll just be printed verbatim. Reading up on the whys of this change, I do have to agree with it. We can add additional extensions if needed, but I do have to concur with the developers on this adjustment: it makes sense.
Tech explanation of the why of the change:
[img]http://forumname.com/forum/login.php?logout=true[/img]
That little snippet in someone's img tag would just spit out a broken image tag, and also log the viewer out of the forum. Not exactly a desirable effect. This is just one of the tamer possibilities available, I can think of much more interesting end-results.