Hello,
There are two places where a person can make a picture appear automatically on their posts. The first is an avatar, which appears just above the contents of your message, below your name. My avatar is a B&W picture from the 30's. However, there is a 128x128 pixel restriction on those, so you're not going to get much detail into the avatar.
A more likely alternative is to put a picture in your signature, which appears at the bottom of every post. For instance, this guy:
http://www.classicoldsmobile.com/for.../64rocket.html
has a picture of his car as part of his signature. To customize your signature, log into the forum, click on the "User CP" menu, then on "Edit Signature". At the bottom of that page, you can upload a picture. Also note that you can make a "Profile Picture", but that picture only appears if people are looking you up specifically, not in your posts.
See the forum FAQ for more info.
Mister