@boofo
first of all this code is nothing to do with guests. this code is only to stop other sites from hotlinking images from vbulletin forum. as all the images are served through attachment.php in vbulletin , so we need to protect our attachment.php abuse.
if any other website or forum will place any images from my website , for example this image url
http://www.nidokidos.org/attachment....3&d=1304560281
which is serving image through attachment.php , using this code in htaccess will not let the images of your site being displayed on the other website , but it will replace your website banner.
so in a result they will remove your images from their website. the code is working perfectly fine with me.