Forcing people to host their avatars elsewhere would remove a large chunk of the traffic.
The other (more expensive) option would be to use someone like Akamai for distributed caching. We use them at work and they handle >95% of our daily traffic (about 3m page views/day, 450gb/day).
2.3Gb/day sounds very high for a site where most of the images should be cached pretty quickly. What about entirely separate hosting for the uploaded images. I'm sure this could be coded in, and would take a lot of the strain off the BB server.
Good luck.
Ali