Many thanks to you tepatria.I would say "This painting is nothing but a copy..." but your sentences are understandable and I would not mark them wrong. It is quite a strong way of saying the painting is not an original. It is a copy or it is fake - if you are trying to pass it off as the real thing.
Many thanks to you Floise.Hi Philanguy,
If you say that something is nothing less than ...., you mean that it is equal to....
Her performance was nothing less than brilliant = Her peformance was brilliant.
Their workmanship was nothing less than perfect = Their workmanship was perfect.
What grammar book are you using?
When I read the original post, my first thought was that the sentence should go like this: "This painting is nothing more than a copy."It cannot be less than a copy or more than a copy.
So the error is semantic rather than grammatical.