hit FAQ up above.
or
The current thought on preventing sites like Fusker/UsefulIdiots/etc from hotlinking your graphics is the following set of rules in your .htaccess file.
If you want to disallow the empty referrer, you can remove the first line of the rule with the !^$ in it.
RewriteEngine on
# leave this line in allow empty referrers, remove to disallow empty referrers
RewriteCond %{HTTP_REFERER} !^$ [NC]
RewriteCond %{HTTP_REFERER} !^
http://(.*@)?([a-z0-9-]+\.)*yourdomain\.com(:[0-9]+)?(/.*)?$ [NC]
RewriteCond %{HTTP_REFERER} !^
http://(.*@)?1\.2\.3\.4(:[0-9]+)?(/.*)?$
RewriteRule .*\.(asf|mpg|mpeg|wmv|avi|rm|gif|jpeg|jpg|zip)$ - [NC,F,L]