As an example, here's the one I use:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www.)?solareclipse.net/ [NC]
RewriteRule .(gif|jpg)$ http://solareclipse.net/stolen.gif [R,L]
Yes, this does work - the .htaccess is in a subdirectory, so stolen.gif itself isn't filtered. If you want to protect your entire site, you should be able to get away with creating a subdirectory, then .htaccessing it with RewriteEngine off.