mod_hotlink is some software I wrote ages ago to deal with the problem of how to politely* prevent one site from linking to images on another, without using tons of CPU (as does mod_rewrite solutions)
From the README:
Protects non-text content against “hotlinking” – a method for stealing
bandwidth by linking images and content from other websites to present
them as if they were their own.
For Apache 1.3, fully tested with 1.3.26, also works with later
versions. Not tested and not expected to work with 2.x without some
effort. Finally got around to creating a project for it, so hopefully
if it is found to be of use, a group effort will make something more
Please note: The Referer header sent by browsers is completely
optional, and often untrustworthy. There is very little anyone can
do about that. So, this won’t protect you from all hotlinking. In
practice, it works for most browsers.
* Politely as in people can still do it if they disable sending of referrer information, of course.