So how do i do it?
Your not allowed, they saw your avatar, and said no way this kid is getting a picture !!
At the top of the page, click "profile", then in the box on the left side click "profile information". Scroll down to the "Signatures" box and using the [img] tags type in the URL of the picture you want. They have to be pretty small, so don't try to put some big gaudy thing in there as it won't let you.