this is mine which i have used for years, works great, use .gif for the deny picture!
Code:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://.*websitename\.com.*$ [NC]
RewriteCond %{REQUEST_URI} !^.*leech\.gif$ [NC]
RewriteRule ^.*\.(swf|bmp|gif|jpg|png|jpeg|avi|wmv|mpg|mpeg|wav|mp3)$ http://www.websitename.com/leech.gif [NC,R,L]