Uhm, just deny no referrers, people. This isn't rocket science. This is entirely off the cuff, so if it shits on you, sorry.
RewriteRule %{HTTP_REFERER} ^$
RewriteRule .*\.(gif|jp(?e)g)$
http://yourserver.com/no.gif [NC, R]
[Tard Edit: As mentioned above, this will fuck people who don't send referers, but they generally do this on purpose..]