I did a few derivatives of either Vincent Van Gogh or Franz Marc. I signed them but also put on "After Van Gogh" or "After Marc." I never really considered them originals, even though I tried to do them my way. But then I have seen a Franz Marc painting that was clearly derived from an old frieze of some donkeys...so it is hard to say. I would only use another artist's painting if it is considered in the public domain as the copyright has expired due to the passing of 75 years after the artist's death, but would still give the proper credit to the other artist...like remaking a song. But then if I were to do a painting of the earth from outer space, unless I build my own rocket and take my own photo from space, I must rely on someone else's photo, so is that still considered derivative or infringement, or can I call that painting original?