In order to post pictures on the bulletin board you have to link to them using "bbcode". There's a link explaining what bbcode is here
http://forums.luvtruck.com/faq.php?mode=bbcode which is also available in the lower left corner when your making a new post or posting a reply (BBCode). The link is pretty small so I'm not surprised you didn't see it. The short explanation is that you'll be using the "Img" button instead of the "URL" button.
The only other trick to it is to have your pictures hosted somewhere on the Internet where you can link to them from here. Some ISP's give you 10MB free web space with your account, and some sites offer free picture hosting. The free sites are getting fewer and farther between. If you want to go that route, maybe someone can recommend one? I don't know of any off hand.
So basically it's a two step process:
A) put your pictures somewhere on the Internet
B) post here and link to those pictures using bbcode