British artist David Hockney (R) and Celia Birtwell, who is depicted in the portrait, pose in front of his double portrait, which he painted in 1971, entitled 'Mr and Mrs Clark and Percy' at the National Portrait Gallery in central London October 11, 2006. The exhibition entitled "David Hockney Portraits", showing a comprehensive collection of Hockney's portraits, opens to the public on Thursday and runs until January 21, 2007. REUTERS/Dylan Martinez (BRITAIN)