well from what I get out of the 2256 law which tells you what needs and doesn't need a 2257 record. It looks like you are safe if you don't host hardcore thumbs and stick to NN, faces and possibly even breasts.
What i get out of it even fully nude breasts are fine as long as the model isn't grabbing them in a way to be perceived as sexual. Also IMO going by the book content such as Met Art style sites could legally have a leg to stand on by calling it artist photography. I'm planning on keeping my thumbs I'll just stop cropping full nude or in a way that is considered sexually explicit.
Then I'll just start building more text sites with a small amount of thumbs. I don't do much with galleries so it won't affect me in that area.
This is part of 2256
http://www4.law.cornell.edu/uscode/18/2256.html
(2)
''sexually explicit conduct'' means actual or simulated -
(A) sexual intercourse, including genital-genital, oral-genital, anal-genital, or oral-anal, whether between persons of the same or opposite sex;
(B)bestiality;
(C) masturbation;
(D) sadistic or masochistic abuse; or
(E) lascivious exhibition of the genitals or pubic area of any person;
so by looking at that law, 2257 records are not required if you are showing even full nudes in a non "sexually explicit conduct"
Of course what is "sexually explicit" is a opinion.. But I think they would have a hard time getting a conviction if all you showed was nude poses or better yet torso up thumbs.
That's just the little GFY lawyer in my head talking, but IMO 2256 put's it pretty clear what needs and dosen't need a 2257 record.