Go to "profile" (at the top), forum profile info (on the left).
This is where you can add a picture for your avatar (in the first section) and your siggy (towards the bottom under "signature"). Just remember to check the sizes of any pictures you add. Avatars need to be no larger 100x100. Siggys should also be reasonable so as not require people to scroll their screen and also a reasonable size so it doesn't take a lot of time for people on dial up to load the message. For example, my siggy is 480x321 and 28K bytes.