Simple, at the top of your screen is a "My controls" option, click that.
Down the left hand side is an "Edit signature" option.
Basically then its just the same screen as when you post, you add a pic by clicking the IMG button and then typing the link to your pic in the box that pops up. If your image isn't online somewhere you will need to upload it first.
Does that help?
PS: That is a COOL avatar