Arkady is correct about even using just a small piece of a photo that is copyrighted or has copyrighted elements in it. But also know that photos used for references in artistic pieces of work are protected to a certain extent. Many artists create their work from references of photos and then they sell their art without permission of anyone.
Here's some great reading from The World Intellectual Property Organization (WIPO) website about using copyrighted materials:
Legal Pitfalls in Taking or Using Photographs of Copyright Material, Trademarks and People