Artists copying Artists is business as usual. See for instance Marcel Duchamps "L.H.O.O.Q." or that other one he called "Pharmacy"; don't remember what art work he used as basis. It's sampling.
As I said, I should be allowed to do this. I see "impersonations" of famous works, mockeries, parody, where you can easily tell which famous work is the subject matter; I don't think artists need permission to do this.
Really, if I sell that drawing of mine where I "emulated" Giorgio de Chirico's painting, I'm going to have someone from his estate threatening legal action?
And photographs on the internet--how would anyone be able to tell which photograph, specifically, I took from? What would a collage artist do?