A photo portrait of Sir Elton John, David Furnish and their two teenage sons has been unveiled by the National Portrait Gallery. US artist Catherine Opie took the photo of the singer, his husband and their sons Zachary, 15, and Elijah, 13 - along with their pet Labradors Joseph and Jacob - in the library of their home in Windsor. It shows Sir Elton seated in a purple suit and pink tinted glasses, Furnish in a green suit, and their boys in black T-shirts, one perched on an arm of the sofa and the other on the floor holding a dog toy. The London gallery said it would join its collection of "portraits of the people who have shaped the history and culture of the UK". The image has been hung in a room with portraits of other celebrities including Sir David Beckham, Dame Judi Dench, Amy Winehouse and Letitia Wright. Sir Elton and Furnish said in a statement: "To have our family photographed by Catherine Opie and on display at the National Portrait Gallery is a huge honour.…