Go to user cp located at the top beside log out and look on the left side of the screen and it will say edit signature, then find a picture online etc... then right click on it and click copy image url or copy image location and go back to your edit signature and clik add image up at the to of the editor or just use
Read the rules, just don't make it unreasonably big. Just use your judgement, probably not too much bigger than mine. btw, there can only be one picture as your sig, and it can not be nudity/pornography of any kind.