Click on your name, when you come to your profile press the edit button. In that screen go to signature, and then put in the code from a picture sharing website, most of us use photo bucket, once you put the image code and press the save button and voilà you not have your signature. You can't just put any picture though there are size restrictions, there are lots of people around here that would be happy to make a banner for you, or help you with the banner. Although most people typically don't get a banner or have one until they join a squad