Probably there is a lawyer or two on this forum somewhere who could answer your question directly and I hope they will.
In my own limited experience and understanding, the copyright attaches to the art work itself and any direct and exact reproductions thereof. If a sculptor produces a figure based on but not intended as a copy of a copyrighted illustration or a copyrighted photo, I believe that's OK.
Having said that, if you know the copyright owner or holder, it's just good business and common sense to contact that person in advance, tell them what you intend to do and ask permission. The British Blue Cross Society (a charitable group for the prevention of cruelty to animals) holds the original and the copyright rights to a painting by Fortunio Matania, "Goodbye Old Man". Matania gave them the painting after the war. See the photo. I wrote to them and asked permission to do a figure based on it and immediately received a courteous and thoughtful reply granting approval, even to the extent of producing a commercial model kit version, which I never did.
All the best,
Dan