Sounds like an interesting concept.

I guess it would have to be a separate attachment feature just for the signature editor that would need to limit the size of the graphic based on your signature requirements, if the member doesn't do that in the first place (assuming there are signature size limitations in place). I wonder if automatically resizing the pic for multiple members would have an effect on the performance time of the site?
(I thought I read somewhere on vBulletin.com about the resizing thing, but I can't remember right now....)