William Koch (one of the notorious conservative donor Koch Brothers) bought the 2×3″ ferrotype taken by an unknown photographer in Fort Sumner, New Mexico in late 1879 or early 1880 at a Denver auction last Saturday.

This carte de visite image, commonly referred to as the Upham tintype (named for its longtime owner Frank Upham, a nephew of the original owner Dan Dedrick, one of Billy the Kid’s outlaw friends) is the only image of the famous Western gunfighter believed by experts to be authentic.

Wikipedia article on the Kid, which discusses the photograph.

New Mexico Tourism’s detailed discussion of the photograph.