Never-before-seen images of Frida Kahlo taken by her lover the photographer Nicholas Muray are but a few of the highlights of Sotheby's upcoming Photographs auction (5 April, New York). Ahead of the diverse sale, Sotheby's Specialists chose their favorite images, which include El Lissitzky's pivotal Pelikan Tinte, as well as iconic photographs by Irving Penn, Alfred Stieglitz and Richard Avedon. Click ahead to discover all of their choices.