No worries, it took me a few tries to get it figured out as well. Basically all you have to do is go to profile, scroll down to signature(I assume you knew that much already, but oh well). Then all you have to do is put it in as you would an image in a post. example:
Keep in mind though that this counts towards the character limit for you signature, so if you want text as well, this will limit how much space you have left for it. It took me quite awhile to get the the comment of the j-list ads to fit into my signature without getting rid of other stuff I wanted.