Disallowing EVERYBODY from entering base galleries, but stil

I'd like for gallery to only allow users who logged in to enter the galleries, however i'd like EVERYBODY to be able to view the highlight image and the description.

Possibly in the future i'd like to be able to have EVERYBODY view the highlight image of each gallery then be able to enter the gallery, read the captions but not be able to open up the preview or fullsize image without logging in. Sort of a preview mode until you're logged in.

I know this is possible but reading through the gallery scripts is not easy for me.

thanks