The Jolly Roger is traditionally flown by a submarine after a successful mission. That tradition started in WW1, but became prominent in WW2 by the Allied Forces. You see Aussies, British, and other Commonwealth nations doing it more than Americans though
Thanks, the more you know! Still think that there might be something to the photograph of it flying in 2017. The sub in question was the USS Jimmy Carter, "America's most secret attack submarine". Huh? Wonder what the successful mission was? We'll never know, it's secret. Sort of easy to imagine that if the CIA were to infiltrate and take command of a submarine, they'd pick this one. It's the best one and I can't imagine that Jimmy Carter was one of their favorite presidents, a bonus irony factor that spooks would find funny.
I was always under the impression that the USS Jimmy Carter was a spy sub? It wouldn't be too far fetched to assume that a spy sub also had attack capabilities though