Well, right now the forum's own attachment feature is disabled because it was causing problems with our new Web host. (It will be reinstated, though, once we switch over to the new forum system, which will hopefully be soon - I keep getting sidetracked by other things.
)
For the moment the only way to include an image in a post is to upload the file to your own Web space and then use the YaBBC image tags like this:
[img]http://www.yourhost.com/theimage.jpg[/img]
