PHOENIX (Reuters) - The U.S. Secret Service is investigating the origins of a photo on an Arizona police officer's Facebook page that shows a group of armed youths posing in the desert with what appears to be a bullet-ridden image of President Barack Obama on a T-shirt. Max Milien, a Secret Service spokesman, said on Friday that the agency was looking into the picture taken of seven youths, four of them toting guns, that turned up on the Facebook page of Sergeant Pat Shearer of the Peoria, Arizona, police department. ...