I assume you are talking about a "signature" that is a graphic that goes below all of your posts? At the top of the page click on the profile button. On the left side of your profile page there is a link called "forum profile information" click that. Scroll down and there is a section called "signature". You first have to host the picture on a web site like imageshack or photobucket and then insert the web site address of that hosted picture in the signature section. that's it.