if you could control the size of an image for the avatar and people linked to them from outside an outside url i really dont see the problem.
i've seen upload limitations and size limitations scripts before so i believe you can only link pictures that meet certain requirements.
ofcourse there are drawbacks as people have pointed out but this is a hack that if controlled correctly can work for some forums.
if your already allowing custom image avatars i don't see the problem.
infact i think alot of people out there would love a more refined and controled scripting system for managing peoples pic sizes in both the avatar and the sig.
