attachment

Thanks to firefly, I am able to display attachment on my front page as thumb. However, when I select that guests cannot view attachments, it will display a broken image, which is normal. Is there a way to include in global.php that if user=guest and display attachment=no then if there is an attachment, display guest.gif? Then I will make an image guest.gif stating that only members can view this image. It will display that as big attachment and solve the broken thumb.

[Highlander]

where is the thumb hack?