2256 tells you what needs a 2257
http://my.execpc.com/~xxxlaw/18%20USC%202256_text.html
I would think by the letter of the law those pics would not need a 2257 as they could be considered erotic art. However in reality who really wants to take the chance?
I'm going to be working with a photographer in the near future and that's the type of content we will be shooting. However I don't think I'd even risk not having the full 2257 info.