I don't really know. I'm trying to follow this article on how to resize and post pictures, but other than the last post that I finally got a picture on, it has been a lesson in futility.
As said by Speed, make sure there are no spaces. It should be [IMG]"yourpicturelink"[/IMG] with no spaces between the] and the other [. You seem to be getting it, just might be your spacing. I also found that using the website recommended http://s69.photobucket.com/ works out really well to house your photo's also