Force user to registration to view gallery

I'm developing an app where artists can post their portfolio, including their art. Now I'm unsure what to do in this case.

Example: An Artist produced (artistic) nude tattoo photos / artworks / phptos, etc. which is (at least by law) forbidden in some regions to show this to underage persons (21, 18, 16, whatever).

Should I..

a) force the user, if he/she want's to visit the gallery, as there could be such content or

b) implement an option, where the artist has to state, if there's 18+ material on the gallery and only then force the user (age check link. [but most of us had the experience clicking the "Yes, I'm 18+ y/o"-Button]) or

c) ignore this and get sued?

Therefore I'm interested how the user would react to this obligation? Are there alternatives?