Itsacon

If your server runs PHP, you could use that.

I had a similar problem, and I solved it by having all images served out by a php script (aptly named image.php), which has a list of allowed addresses and hostnames, and only gives out the image if the referer is from one of those domains.
The images themselves are stored in a directory on the server that is not directly accessible by the webserver, only by PHP.

If you're interested I can post the code for the script.

__________________
"The biggest problem encountered while trying to design a system that was completely foolproof, was, that people tended to underestimate the ingenuity of complete fools."
---Douglas Adams