I discovered that the vast majority of the bytes I was serving was due to sites that hotlink, that is, sites that use image src URIs that refer to my site (thereby stealing my bandwidth), rather than copying the image to their own site and serving it themselves. Using Apache's RewriteEngine, as described in this article, allows me to refuse to serve images referred by any page not my own.
Hopefully this will put a stop to the practice. I currently only FORBID the request, but I may serve up some obnoxious image instead. Not yet sure.