A photograph of a smirking Billy the Kid, taken outside a New Mexico saloon near to where the famed outlaw was shot dead, could fetch up to $1 million at an auction on Saturday. The two-by-three-inch (five-by-eight-centimeter) tintype is the only known adult portrait of the Wild West gunslinger who variously went by William Bonney, Henry Antrim, Henry McCarty or just \"the Kid.\" It has also been one of the most commonly reproduced images of the outlaw since it appeared in Sheriff Pat Garrett\'s book on how he tracked down and killed the Kid in 1881. \"This is it -- the only one,\" said Brian Lebel, auctioneer for the 22nd annual Old West Show and Auction to be held in Denver, Colorado. \"We\'ll have over 500 (people) in the audience. We have eight telephone lines and we have three different Internet hookups, so people can bid live online with real-time bidding (and) with a camera, so you can see the auctioneer.\" The unidentified photographer originally made four identical tintypes, but the other three have been lost. The one remaining tintype is owned by brothers Stephen and Art Upham of California and Arizona, who last displayed it publicly in a museum in Lincoln, New Mexico, in the mid-1980s. It has been sealed in a nitrogen-filled envelope and kept in a safety deposit box since then, Lebel said. Billy the Kid\'s jawline appears asymmetrical in the photograph, possibly because he made a face or moved his head during the long exposure time. The image is smeared across the hips -- apparently from the gunslinger stuffing the tintype into his pocket before it dried. For years, Billy the Kid was thought to be left-handed because the tintype shows him wearing a holster on his left side, inspiring the 1958 film, \"The Left-Handed Gun.\" In fact, the holster was on his right side, and only appears to be on the left because tintype photography creates a mirror image of the subject. Lebel plans to start the bidding on Saturday at $150,000, half of what he expects to be the minimum sale price of $300,000. But Western photography collector Bob McCubbin of Santa Fe, who plans to attend the auction, expects bids to go as high as $1 million (700,000 euros). \"This is the Holy Grail of photography,\" McCubbin said. \"I cannot think of another photograph in the entire world that is as famous as this.\"