If it's being hosted on a unix machine, you can use apache's mod_rewrite to put some directives in your .htaccess file. Here's a generator for making a .htaccess file that blocks hotlinking.
http://www.htmlbasix.com/disablehotlinking.shtml
If it's windows and IIS, I don't know how to do it, but I know it'll be a bit tougher.