whowantstoast I don't think they are saying literally improve on Mother Nature's works.
I think what Frith is getting at is that painters are less constrained with regard to many things. For example in photography we can minimize the background, to a great extent with DOF and other choices, but in contrast a painter simply never adds the background.
Another example is perspective, any single shot with a camera only gets one. Painters have no such constraint.
These constraints are imposed by the tools we have chosen.