Click your character name or otherwise go to the Character Administration page (the one where all your skills are listed). At the top of this page there are options, choose the one that says [Edit Lorenzas Atticus' Profile].
You'll find the option to update your character description, biography and signature there. An easy place to find all the HTML codes is right here in the forums when you make a post.
I've mad it to that point but when I try and apply a picture to my sig it still shows up as the link code where the pic should be. How do I get the pic to stay there?
So. Lets take mine for example. (I have a php rotator.)
The link to the image(s) is http://www.darklordsofwargames.com/Lex/signatures/avatar.php/AvI_sig_1.png