A photo showing Billy the Kid and his partners in crime playing croquet was bought for dirt-cheap on a blea market and later turned out to be worth a fortune. Who is this Billy guy, and why is this photo so sensationally expensive?

A photo showing Billy the Kid and his partners in crime playing croquet was bought for dirt-cheap on a blea market and later turned out to be worth a fortune. Who is this Billy guy, and why is this photo so sensationally expensive?