To edit your siggie from the message boards, choose the "My Settings" link at the top of the forums (after you login).  The "My Settings" link is right below the banner ad.  Then click on "User Settings" to expand the list of choices.  You can then type into the "Signature" text box.  To preview, click the page/magnifying glass icon. 

If you want an image in your siggie, paste in the HTML tag.  (For example, in Photobucket you can get this tag right above the IMG code tag).

You can also upload an avatar from the "User Settings" options as well.