What is believed to be the only surviving, authenticated portrait of Billy the Kid fetched more than 2.3 million dollars (£1.4m) at auction in Denver, Colorado.
The tintype went to private collector William Koch at Brian Lebel's 22nd Annual Old West Show & Auction, where auction spokeswoman Melissa McCracken said the image of the 1800s outlaw was the most expensive piece sold at the event
A 15% fee was tacked on to the bidding price, making the actual selling price more than 2.6 million dollars (£1.6m).
Organisers had expected it to fetch between 300,000 and 400,000 dollars (£188,000-£251,000) .
The tintype is believed to have been taken in 1879 or 1880 in Fort Sumner, New Mexico.
It shows the outlaw dressed in a rumpled hat and layers of clothes, including a bulky sweater. He is standing with one hand resting on a Winchester carbine on his right side and a Colt revolver holstered on his left side.
Tintypes were an early form of photography that used metal plates. They are reverse images, and the Billy the Kid tintype led to the mistaken belief that the Kid was a lefty. The myth inspired the 1958 movie The Left Handed Gun, starring Paul Newman as Billy.
Billy the Kid gave the image to a friend, Dan Dedrick, and the tintype has been owned by his descendants, the Upham family, ever since. It has only been publicly displayed during the 1980s at a museum in Lincoln County, New Mexico.
Ms McCracken said it was recognisable around the world as a classic image of the Old West.
"There's only one photo of Billy the Kid and I think that's why it captivates people's imagination," she said before the auction.
The tintype was sold along with more than 400 other Western-themed items, including documents from Buffalo Bill's aborted divorce, Native American antiquities, and a painting from Andy Warhol's Cowboys and Indians series depicting a Navajo woman with a baby on her back.