##
# Allows only specified servers from linking to images
# Adjust to your own needs
##
location ~* \.(gif|jpg|jpeg|png)$ {
    valid_referers none blocked ~.google. ~.bing. ~.yahoo. ~.facebook. ~.fbcdn. ~.ask. server_names ~($host);
    if ($invalid_referer) { 
        return 418; 
    }
}