Different permissions for thumbnails vs photo in albums?

[TNCclubman]

I want the album only viewable by registered people, but the thumbnails viewable by all (no restrictions)

Is this an easy file edit to do? (I dont think its a changeable option in the admin cp)

[TNCclubman]

Any ideas? It would be a good feature to have since the thumbs are used to entice people to join to view the full picture. As it is now, no one can see the thumbs unless theyre registered. (which being registered to view the full photo is what i want)