IIRC a part of copyright "activating" is publishing the content in the first place, so I don't think GP would have a good case to begin with, not that it matters.
Copyright does not require public publishing of the content — just "fixed in a tangible medium of expression". This would have been accomplished when the painting was created on paper.
This is essentially what I was referring to in the second sentence. Even under the current system you might theoretically have a claim, in practice you'd have a hell of a time proving it, so it wouldn't tangibly change the outcomes like OP is suggesting.