Also, not just the signature picture ... but pictures can be put into actual posts too ... to do that you just use the [img][/img] tags ... you simply put the url of the image you want to post between the img tags ...
ex. [img]http://www.com/someimage.jpg[/img]

check out more forum codes here
http://www.utadanet.com/modules/newbb/viewtopic.php?topic_id=52&forum=9


As for making promo images for the site... you can just post them up in the forums and send me a Private Message