|Organisers said they had expected the tintype could fetch between $300,000 and $400,000 [Reuters]
The only surviving authenticated portrait of Billy the Kid, the 19th-century Wild West outlaw, has sold for $2.3m.
The tintype, an early form of photography using metal plates, was put up for auction at Denver on Saturday and went to private collector William Koch.
Auction spokeswoman Melissa McCracken said the image was the most expensive piece ever sold at the event.
Organisers had expected it could fetch between $300,000 and $400,000, the AP news agency reported.
"There's only one photo of Billy the Kid, and I think that's why it captivates people's imagination,'' McCracken said before Saturday's auction.
"It's recognisable around the world as a classic image of the Old West."
Believed to have been taken in 1879 or 1880, the tintype shows the notorious outlaw dressed in a sombrero hat and layers of clothes topped with a bulky sweater.
His right hand rests on a Winchester carbine while a Colt revolver is holstered on his left side.
Billy the Kid gave the image to a friend, Dan Dedrick, and the tintype has been owned by his descendants ever since.
It has only ever been publicly displayed once, during the 1980s at a museum in Lincoln County, New Mexico, where Billy the Kid became a legendary gunman in the 1870s.
Only 21 years old at the time of his death, Billy the Kid had grown into a legend because the governor of New Mexico had put a price on his head for killing up to 21 people.
He was shot dead in 1881 and buried in Fort Sumner, New Mexico.