As it stands at the moment, signatures only allow for text and url links. If pictures in signatures are something that people would like, that may be something I bring up to the admin.
The only lingering concern that has made it not a default option is the abuse it can bring about, as I am sure all of you have also noticed on other forums when scrolling through five posts takes far too long because everyone has gigantic signatures.
But, again, if some sort of image signature is something people are missing, we should be able to get something arranged.