Your description describes the problem pretty well, and the "problem" lies in the "unknown number of hits" coupled with the filesizes of the thumbnails and banners.
Yes, on both counts. The Control Panel-->Status-->Bandwidth screen will show you your usage (updated daily) and allow you to "throttle" bandwidth in order to help control your usage and resultant charges for extra bandwidth. As it only updates once a day, unless you set the bandwidth to throttle such that you wqill allow for a "heavy day's" worth of traffic you could still go over - but with careful planning you should be able to stary under your quota or fairly close (within a few GB, and at $.50 per GB, that's not so bad!)
There is a lot more about all this on this Dreamhost Wiki page on Bandwidth.