To edit your signature go to the top (green) menu:
Member profile -> Profile edit
Click the "Forum" tab.
The big text box is for your signature.
Here is my signature in "plain text"
[url="http://www.link_to_your_website.com"][img]
http://www.link_to_your_signature_image.jpg[/img][/url]
That is what I have used in my signature
