Perhaps the most infamous music deal ever struck involved no contracts and no lawyers. The blues singer Robert Johnson, so the legend goes, acquired his unearthly musical talent after meeting the devil at a crossroads.

Until now, there were only two verified photographs of Johnson (1911-1938), who remains the most inspirational musician produced by the Mississippi Delta and the man Eric Clapton once anointed as "the most important blues musician who ever lived". This weekend a third, newly cleaned-up and authenticated image has been released by the Johnson estate showing him standing next to musician Johnny Shines.

Forensic work on the photograph began in 2007, when Lois Gibson, who works with the Houston police department, analysed the features of the long-fingered figure holding the guitar. Gibson, who found the identity of the sailor kissing the nurse in the Life magazine photo of Times Square on VJ day the second world war ended, has ruled that "it appears the individual is Robert Johnson. All the features are consistent, if not identical." The only differences, she added, were due to the angle of the camera or the lighting.

The new photograph came to light eight years ago, when a classical guitarist called Steven "Zeke" Schein was searching eBay for an old guitar. He spotted a thumbnail picture with a caption that read "Old Snapshot Blues Guitar BB King???" and bought it. On inspection neither man in the photograph looked like BB King, but Schein noticed the length of the man's fingers on the guitar and the way his left eye was narrower than his right.

One of the other two known photographs of Johnson is postage stamp size and is thought to have been taken in a booth in the 1930s. It was first published in Rolling Stone in 1986, the year that Johnson was inducted into the Rock and Roll Hall of Fame, and shows him in a button-down shirt, staring directly at the lens. A cigarette hangs from his lips and his long fingers rest on a guitar neck.

The second image was taken at the Hooks Bros photographic studio in Memphis. In it, Johnson sits cross-legged on a stool with his guitar, wearing a pin-striped suit and a tie. This portrait was used on the cover of Robert Johnson: The Complete Recordings, the two-CD boxed set issued by Columbia Records in 1990.