Photographer Jill Greenberg regularly photographs celebrities for advertisers. In her new book, though, she focuses her lens on primates. Specifically, Greenberg has compiled monkey images and put them in her book "Monkey Portraits." Greenberg, who also has taken portraits of bears, allows the monkeys to take on humanlike expressions in the pictures. Frustrated, smiling, sexy and sad, these portraits may give a glimpse into the animals' thoughts.