By Joe Brock PRETORIA (Reuters) - A South African prosecutor forced Oscar Pistorius on Wednesday to look at a forensic photograph of the head of his girlfriend Reeva Steenkamp after it was destroyed by a gunshot fired by the Olympic and Paralympic track star. In a dramatic opening to his cross-examination of Pistorius, prosecutor Gerrie Nel made him say that he had killed Steenkamp then later confronted him with the photograph showing the side and back of her skull matted with blood and brains. "Take responsibility for what you have done," Nel said, drawing gasps from the public gallery and causing Pistorius to bury his head in his hands in the witness stand, rock from side to side and weep. The double amputee sprinter, once revered across the world for his triumph over adversity, faces life in prison if convicted in the Pretoria High Court of the murder of Steenkamp, a 29-year-old law graduate and model.