They don't allow users to attach any images to threads?!?! Errr... That's pretty odd
IMO to not allow them, and to charge people for it.
If it was some massive forum and they had bandwidth issues, then yeah maybe some pay scheme would be in order, but they only have 4,000 registered members and currently < 100 people online?
They really need to upgrade to the latest vBulletin
BTW, they are quite a few versions behind.
As for tips. When allowing attachments, you can limit the amount per person in vBulletin (though most forums set it to unlimited, since it would mean people would have to delete old attachments in order to post new ones). Also, I don't allow unregistered users to view images (that saves on a good amount of bandwidth), and also I have set in my robots.txt file for the google-imagebot to NOT index the site (it just indexes images, again a bandwidth hog). Lastly I don't allow hotlinking so that also prevents bandwidth leeching. (hotlinking is when site B directly links to some image on site A).
Lastly, if they are having that many problems with bandwidth, probably time to find a new hosting provider. (I never visit hexus.net so I don't know what their situation is).