What was I wrong about? the <img src="http://path.to.image"> tag works in the signature without problems. HTML is allowed in signatures, which is why scrolling text and other things people put in their sigs work.

And to upload an image for your avatar, you need to be a Registered Voter. Your account is automatically upgraded at 500 posts or if you have 250 posts and 5 wins, you can ask an admin to do it. The 100 posts thing may have been from vb2.