I'm new here but I've just had a quick look and the forum looks like it's using php.
Now from memory to upload from desktop the board owner had to implement a plug in.
It looks like this either hasn't been done or they have to authorise each individual account (just a guess).
Otherwise the only way is to have your own images saved to a hosting site such as Flickr and then paste the url between the HTML code [img]your%20url[/img]