JOHANNESBURG — South African police are investigating an image posted on Facebook of a white man with a hunting rifle and a big grin kneeling in a classic hunter’s pose over his “prey” — what appears to be the lifeless body of a black boy. The picture was published on the front page of South Africa’s Sunday Times along with a police plea for the man in the picture or anyone who recognizes him to help their investigation. It is not clear if the picture is authentic, or whether it has been digitally manipulated.