Buried treasure isn't a legend; sometimes it's a reality, buried in an antiques store in Fresno.

Randy and Linda Guijarro were browsing a shop in the Tower District when they stumbled upon two cardboard boxes of old photographs. Randy Guijarro picked three he liked, paid $2 and went on his way.

Little did he know, a man in one tintype is reportedly William H. Bonney — Billy the Kid. At the time of his discovery, there was only one known photograph of Billy the Kid. It sold at auction for $2.3 million.

This find is valued at $5 million.

"It's the holy grail of not just western photography. It's the holy grail of photography," said Jeff Aiello, executive director of a new Billy the Kid documentary. "It is the rarest photograph in the world and it was found in a Fresno junk shop."

The Guijarros bought the photo in 2010, but it wasn't until recently that the image was finally authenticated by Kagin's Incorporated in Tiburon. The photo is believed to have been taken in New Mexico around 1878 when the Kid would have been about 17. In it, he's playing croquet with his fellow Regulators, a group of ranch owners and cowboys who were deputized during the Lincoln County War.

The Kid was dead three years later, shot by Sheriff Pat Garrett in Fort Sumner, New Mexico.

Also present in the photo is Sallie Chisum, whose personal journals provided important first-hand accounts of Billy the Kid.

The National Geographic channel will be airing the documentary 'Billy the Kid - New Evidence' about the discovery on Sunday night at 9.