img BBcode linking instead of displaying picture

[TheGreatTK]

I'm having a bit of trouble on my forum, whereby a user will attempt to post a picture using the [img] tags, but what happens is instead of the image being displayed inline, it's showing up as a direct link to the picture (as if the person has used [url])

I'm sure this is a bogus setting somewhere, but i'm probably missing it. Any ideas where i need to look?

**edit more info**

Also, this behavior happens regardless if its a user or an administrator making the post, so that all but rules out permissions..