John Potter says he was browsing for antiques at a yard sale in south Georgia when he came across an old picture frame containing an enigmatic image -- the dark silhouette of a person in a hat and coat standing to one side and a long, boxy structure looming in the background.

Potter says he didn't have the $175 the owner in Waycross wanted for the photograph, a hazy image further blurred by stains from water or chemicals. He also recalls finding a written clue to decoding the image on back of the frame. The inscription read: "CSS Georgia."

"I knew exactly what it was," said Potter, a Savannah native now living in North Carolina. "I thought, `This belongs in a museum."'

"Believe me, if I had thought that the image was the CSS Georgia, I would have moved mountains to make sure we got it."

- Paul Blatner, museum curator

That was roughly 30 years ago. The only evidence of the mystery image are photographs snapped of the original to share with historian friends back in Savannah. Civil War experts say the image, if authenticated, would be the only known photograph of the CSS Georgia, an armored Confederate warship that was scuttled by its own crew 150 years ago as Gen. William T. Sherman's Union troops captured Savannah.

Experts still have many questions about the sunken ironclad, and they think the original photo could help them find answers.

"The photo is just as much a mystery as the CSS Georgia, because nobody has seen it in years," said Julie Morgan, an archaeologist for the Army Corps of Engineers.

The federal agency is spending $14 million to raise the Confederate ship's wreckage from the bottom of the Savannah River. Divers have been in the water since January and work is expected to wrap up this fall.

In a military sense, the CSS Georgia was an ironclad flop that never fired a shot in battle. The Civil War ushered in the era of armored warships. In Savannah, a Ladies Gunboat Association raised $115,000 to build such a ship to protect the city. But the 120-foot-long CSS Georgia's engines proved too weak to propel its 1,200-ton frame against river currents. It stayed anchored off Fort Jackson as a floating gun battery before it was scuttled in December 1864.

No photographs of the ironclad have been confirmed. Neither have blueprints or construction plans. Several artists drew renderings of the CSS Georgia, but they differ in their details.

If the image Potter found the 1980s could be authenticated, would it necessarily be the CSS Georgia?

Robert Holcombe, former curator of the National Civil War Naval Museum in Columbus, dug up archival information on the CSS Georgia for a report to the Army Corps in 2003. Holcombe said the shape in the photo conforms with known aspects of the CSS Georga's design that made it unique among ironclads -- namely an armored casemate that covered the ship's entire deck with sides sloped at 45-degree angles.

"If it's an original, it's certainly the Georgia, just by process of elimination," Holcombe said. If the photo isn't authentic, he said, "it's an awfully good fake."

The Army Corps is spreading word that it's seeking the original photograph through its website and by using social media. Morgan said she hopes the owner may have other relics related to the ironclad.

Potter said he tried unsuccessfully to reconnect with the photo's owner, who soon moved away from Waycross. He donated a photo of the original image to the Georgia Historical Society, which confirmed it received Potter's gift in March 1986.

A couple of years later, Potter said, he got a letter from a family member of the original image's owner. He said the letter, which he no longer has, claimed the original photograph had been donated to the Savannah History Museum.

Paul Blatner, the museum's curator and director from 1984 to 1990, said he never saw a photograph of any Confederate ironclad during his tenure.

"Believe me," Blatner said, "if I had thought that the image was the CSS Georgia, I would have moved mountains to make sure we got it."