Well I wasn't really trying to figure the cost of those, because you can eliminate that cost by going backwards... it's the increase in membership (that most seem to think would happen) that would be irreversible increases to bandwidth.
HOWEVER... this is a very decently put together forum, no added fluff or bloat... the only images are basic things that appear once, and then, if cache'd will incur no other loads... it's not a graphical site very much, and it's only an occasional forum that requires a screenshot of the application, and that's viewed minimally I would think.
There's ways to hinder bloating up forum posts, like eliminating image tags within the post, file capping the 'additional attachments' and encouraging the use of imageshack and such services, or lily pad to some