That linking format only works in the articles section of the site.
If you want to get your pictures into forum posts (as opposed to into an article), first upload the pics to the Dakka gallery and then copy/paste the code from the 'Forums' field into your forum post.
You can read more in the Dakka
FAQ about how to do this if you still need more info:
http://www.dakkadakka.com/dakkaforum/posts/list/205120.page#206763