Sometimes IE doesn't want to add the trailing slash, so the referer will be
http://yourdomain.com instead of
http://yourdomain.com/
This one will take care of that and everything else....
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)?yourdomain.com(:[0-9]+)?($|/)+ [NC]
RewriteCond %{HTTP_REFERER} !^
http://1.2.3.4(:[0-9]+)?($|/)+
RewriteRule \.(jpg|gif)$
http://www.google.com/ [NC,R,L]