If the images are hosted elsewhere then the browser on the client side is doing the work, the UBB is only serving the text for the browser to use. As far as reducing the traffic generated there are a couple of choices. One would be to utilize the PHP accelerator and the other is to use the UBB compression hack that was in the 6.1 mod forums I believe.
Remember that there are always trade offs and this will increase CPU loads. I have a fantastic suggestion for you though, go to
www.ubbhosters.com and check out their plans. Get ahold of Steve Klenert over there (or at
www.digitalprinceton.net) and discuss this proplem with him. He is great at setting up plans to fit your needs and will give you a realistic price.
If you are serious about this then
click here and give them a holler. The service is outstanding. I don't get commission or anything so you don't have to mention me...I am hosted with him and love the customer service.