okay i dont think none of the posts above is gonna do it. i had trouble with mine also a few months back because i wanted to change it.
try these steps and see if it works.
1) copy the url of the image you want.
2) as you are reading this scroll up this page to the "profile" link. its up there between the Usergroups and No new messages link. click on the profile link.
3) look for the space to type in the url for your sig. now paste the url of the pic with the
after. remember no spaces between the img tags and url.
4) then scroll to the bottom of the page and click submit.
5) then if you click on "your account" and look for your sig. there should be a set of numbers and letters in the img tags.
it should work. i couldnt figure it out before. i pm'd dj, pen, loki and just about all the regulars here, no one knew. then i accidently figured it out.
post up if it works or not. i want to know if i am missing something.