I have a similar problem.

One forum (gallery) is displaying the text link like so...

Code
[img]http://www.example.com/forums/gallery/20/medium/349.jpg[/img]

It displays as an image in other forums.

The image is only about 20Kb and I have 'Max Size for inline uploaded images' set at around 50Kb.

I also have 'Can post using Markup/UBBCode' set at 1.

Anyone see what I'm missing?