It's tough, because on more than one occasion I have done fandom copies of some famous original artwork. However, I never pass it off as anything but a fandom copy, fully linking to the original artist, and I think more importantly it's never traced from a photo. The point is to learn from the style and have some fun doing it. Heck, there's a show on British television of an artist who creates celebrity portraits in the style of a famous artist, which I think is really cool!