British actress Keira Knightley has defended her decision to pose nude for photographer Annie Leibovitz's eye-popping Vanity Fair cover shoot. Knightley, photographed alongside designer Tom Ford and a naked Scarlett Johansson, is proud to have been snapped by legendary Leibovitz - clothed or not. She says, "I'm 20. It's Annie Leibovitz. Why not? It's something to show the grandchildren."