[NSFW] Password protected pages/galleries

Hello all,.
I have a question. On my site I want to make some pages private, I mean, only accessible after entering a password eg.
Visitors can obtain this password, either for a fee or, for example, by subscribing to a mailing list.
So when the visitors click on a photo, a kind of form should open where one of the two options is present.
How can that be solved in the most elegant way?

If I just put a page or gallery private, it sort of disappears for the visitors, they don’t see a thumbnail or anything like that for them to click on, so that’s not what I want!

The page I need help with: [log in to see the link]