What is believed to be the only surviving authenticated photograph of Billy the Kid fetched $2.3m (£1.4m) at an auction in Denver, Colorado.
The tintype photograph was sold on Saturday to Florida billionaire and private collector William Koch at Brian Lebel's 22nd Annual Old and West Show & Auction.
Auction spokeswoman Melissa McCracken said the image of the 19th-century outlaw of the Wild West was the most expensive piece ever sold at the event.
Mr Koch said after the auction that he plans to allow some small museums to display the photograph. "I love the old West," he said. "This is a part of American history." The photograph is believed to have been taken in 1879 or 1880 in Fort Sumner, New Mexico. 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 Billy the Kid was left-handed. 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, where Billy the Kid became a legendary gunman in the 1870s.
Pictures at a price: world's most expensive photographs
1 Untitled #96 by Cindy Sherman. Sold for $3.8m at Christies, New York in May 2011.
2 99 Cent II Dipychon by Andreas Gursky. Sold for $3.3m at Sotheby's, London, in February 2007.
3 The Pond Moonlight taken in 1904 by Edward Steichen. Sold for $2.92m at Sotheby's, New York in February 2006.
4 Kremlin of Tobolsk taken in 2009 by Dmitry Medvedev. Sold for $1.75m at a charity auction in St Petersberg in January 2010.
5 Nude taken in 1925 by Edward Weston sold for $1.6m at Sotheby's, New York in January 2008.