If you go to "account" in the menu bar, then "settings" and finally "more options" you'll find a list of options at the top of the page, one of which is "edit forum signature" (second last option in the far right column).
Here's some more info from the FAQs:
Signatures are restricted to 10 lines of text to keep the site clean looking and fast loading. Images can be up to 500 (w) x 150 (h) px in dimension. Please keep signature images below 75kb out of courtesy to people on slower connections.