In the seven decades since his mysterious death, bluesman Robert Johnson’s legend has grown—the tragically short life, the “crossroads” tale of supernatural talent, the genuine gift that inspired Dylan, Clapton, and other greats—but his image remains elusive: only two photos of Johnson have ever been seen by the public. In 2005, on eBay, guitar maven Zeke Schein thought he’d found a third. Schein’s quest to authenticate the picture, Frank DiGiacomo discovers, only led to more questions, both…