Just a thought.
Most signatures, as far as I´ve seen, were big pictures... Maybe you could set the maximun height for the profile picture to 250 pixels, or something like that, and then encourage posting the pictures there, rather than in the signatures. In the end result it wouldnt seem to take as much space.
Only thing is that though some of their pictures could easily adapt to that size, some wouldnt... but those could probably seek to renew their look.
Anyway, hope you can find a good compromise for all members to be happy.
