The more you post, the more stars you get. As you can see I spend way too much time in here, but I can't help it..! LOL
you'll get your first star when you reach 10 posts, another one at 100, a third at 500 (I think), a fourth at 1000 and the fifth at 2000 posts. The ranking names change too when you reach those numbers - so they do not necessarily reflect your skills as a modeler. Those names (and the level of posting required to obtain them) were suggested by FSM members in the early days of this Forum.
As to adding a pic to your SIG, just go into your Profile, and add the url (http://...) of the photo you want into the SIG field (the one that says text and images I think); You'll need HTLM tags in from and at the back of the URL: [ img] right before it and [ /img] right after, but with no gaps in the tags and between them and the URL.