I'm not sure - but you could try hosting the photo here in the Gallery section or somewhere else like Photobucket, then insert the HTML code into your signature. If that doesn't work then I'm out...
I think you're right Steve. You have to insert it as HTML. I never did try to figure out how to do that, so I just put the image I wanted in my signature, in my avatar instead.
"I thought that because fewer wolves meant more deer, that no wolves would mean hunters' paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view."
Aldo Leopold