On the right-hand blue bar thing, there's an option called 'My Settings'. Click it. There should be an option to arrange your signature. It has a limit of 255 characters.
In order to put a picture in, you'll need to use image tags. So [ img ] [ /img ] without the spaces, and of course, inside, the link to your image.
If it's not attaching to posts after you've done this and saved it, go to a post and be sure the box at the bottom that says 'attach signature' is ticked.