Restrict Gallery Images

Can someone point me in the right direction on how to restrict specific images in my galleries?

I am a model and my galleries have mixed nude and non-nude images. I would like to tag nude images as such and then force visitors to become a member of the site to be able to see the nude images. Or possibly sign up for my email list, or something. At the same time I want them to verify that it is legal for them to view artistic nude images based on the community in which they live.

Ideally, I would have a default image that would appear in place of any nude-tagged image if they have not already passed the verification.

NOTE: I want to do this at the image level rather than the post or page level because I want them to see as much as possible within each gallery.

My site is http://www.ChristopherRyan.com.