A newly discovered photo of Billy the Kid, depicting the outlaw playing croquet, is expected to sell for $5m.

Not bad for an item bought as part of a job lot for $2. It is only the second authenticated photo to feature the American gunman.

Billy the Kid is fourth from the left

Randy Guijarro purchased the piece of US history from a junk shop in California. After closely scrutinising the photo of young men playing croquet, Guijarro presented it to an authentication firm with the fantastical suggestion that this was "the Kid" and his gang, the Regulators.

"When we first saw the photograph, we were understandably skeptical - an original Billy the Kid photo is the Holy Grail of Western Americana," explains David McCarthy, Kagin's senior numismatist.

"We had to be certain that we could answer and verify where, when, how and why this photograph was taken. Simple resemblance is not enough in a case like this - a team of experts had to be assembled to address each and every detail in the photo to insure that nothing was out of place.

"After more than a year of methodical study including my own inspection of the site, there is now overwhelming evidence of the image's authenticity."

Kagin's has put a valuation of $5m on the photo and is seeking buyers.

The 4 by 5 inch tintype shows the gang playing the popular pastime in New Mexico during the summer of 1878.

Taken in the hours after a wedding, it depicts the 19-year-old gunman in a rare moment of leisure. He was already a hardened killer by the time this photo was taken. He claimed to have killed 21 men - one for each year of his life. Many historians put the number at nine.

If it does achieve $5m, it would become the most valuable photograph ever sold. The record is held by Andreas Gursky's Rhein II, which made $4.3m in 2011.

The other known Billy the Kid photo - smaller than the present example - sold for $2.3m in the same year.

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