>But if I took 100 paintings from a single artist and combined them all into new works of art, that would probably be copyright infringement in my view

What makes you say that? A work is considered derivative with regards to another work, not to an author. If we take your jigsaw example and accept that the final result would not be derivative of any of the works that contributed each individual piece, and then pretend as if in actuality all the sources were from the same artist, what would change that would suddenly make the result derivative from some or all of the original works?