The solution outlined in this article requires your site to be hosted on a machine using the Apache web server and that your web host allows you to override the server's configuration using a .htaccess file. For the more technically inclined, it uses the facilities provided in the mod_setenvif Apache module. If this is not the case for your website, you cannot use the suggestions given here. You might wish to check out my PHP solution, How to Protect Your Images from Bandwidth Thieves, instead.