As the docs suggests one should make a hidden album for thumbnails...
But how can I change permission for public viewing for individual albums?
click "properties" next to the album thumbnails and use the dropdown "Album can be viewed by" to decide who can access this particular album. If you don't have this dropdown option at all, go to coppermine config first and set "Users can have private albums (Note: if you switch from 'yes' to 'no' any current private albums will become public)" to "Yes".
Joachim