put the following in a .htaccess file to protect that directory (and all dirs under it) from being hot-linked from other people's webpages.

RewriteEngine On
RewriteCond   %{HTTP_REFERER}       !^https?://(?:.*\.)?parseerror.com/.*$ [NC]
RewriteCond   %{HTTP_REFERER}       !^$
RewriteRule   \.(?:jpe?g|gif|png)$  http://parseerror.com/hotlink-annoying.gif [NC,L]

obviously you'll want to change "parseerror.com" to match your own domain