I grapple with this issue from time to time. I ask myself: "When can I sign MY name to a creative work that I have done?" Here is one answer that I ran across today that some of you might find interesting. To me, it is more succinct than other research that I have run across.
“Question: Artist's Copyright FAQ: If I Change 10 Percent, Isn't It a New Image? Answer: The belief that changing 10 percent of an image means you've created a new one is a myth (as is changing 20 percent or 30 percent). The fair use guideline is that you can use 10 percent of something.
It's certainly not a legal test, but as a rule of thumb consider whether, if your painting were put next to the painting or photo you're copying, would someone say you'd based it on the original? If so, you're risking copyright infringement. Don't fool yourself with this 10 percent change myth.” by Marion Boddy-Evans, Painting Expert - http://painting.about.com/cs/artistscopyright/f/copyrightfaq6.htm