Here is what I use and it always works well, also I always drop it into the public_html dir.
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://yourdomain.(.*) [NC]
RewriteCond %{HTTP_REFERER} !^http://(.*).yourdomain.(.*) [NC]
RewriteRule \.(gif|jpg|jpeg|pdf|mp3|GIF|JPG|JPEG|PDF|MP3)$
http://www.domainname.com/upload-int/nolink.gif [R,L]