Serving up images dynamically and preventing hotlinks

If you're on Apache, use mod_rewrite to redirect access to any *.jpg
file to an error page if the referrer is not your own domain. You can
put the rewrite directives in the .htaccess file, or if you have
access to the main Apache config, in the httpd.conf file. Google for
"mod_rewrite hotlink" and you'll find lots of examples and variants.