The photograph spotted on eBay shows Selina Norris Gray, right, who had responsibility for the Arlington House the Lees had lived in for 30 years. (National Park Service)

When Robert E. Lee’s wife, Mary, fled Arlington House at the start of the Civil War, she gave her personal slave, Selina Norris Gray, the keys to the mansion and responsibility for the grand house the Lees had lived in for 30 years.

Gray fulfilled her duties. She is famously credited with saving from marauding Union soldiers numerous heirlooms belonging to George Washington that were stored in the house.

Now the National Park Service, which administers Arlington House, has acquired what it says is a rare and previously unknown photograph of Gray and, apparently, two of her eight children.

The photograph was spotted last month on the Internet auction site eBay by Park Service volunteer Dean DeRosa. The seller, in England, had found the photo in a box of “unwanted” pictures at a British version of a yard sale.

A Park Service statement said that its nonprofit partner, Save Historic Arlington House, bid on the photograph and, “against stiff competition,” won.

“This is a big deal,” National Park Service spokeswoman Jenny Anzelmo-Sarles said Thursday.

“It’s in­cred­ibly rare to have photos of slaves that we can identify,” she said. “Since slaves were property, it’s really hard to identify the people in images like this. This is a priceless item to add to our collection.”

She said the Park Service is sure the double image, which is identified on the back only as “Gen Lees Slaves Arlington Va,” depicts Gray, the older woman in the picture, and probably her children.

The Park Service was able to compare the new photo with an identified photo of Gray already in its collection. Anzelmo-Sarles said the new photo is believed to have been taken outside Gray’s slave quarters at Arlington.

Arlington House had Washington heirlooms because Mary Lee was the great-granddaughter of Martha Washington, the president’s wife. And Mary Lee’s father, George Washington Parke Custis, had been raised by the Washingtons.

The Union army took over Arlington on May 24, 1861, after Robert E. Lee joined the Confederacy and his family left the mansion, which contained Washington china, furniture, and art work.

Gray tried to keep track of Washington and Lee valuables in the house.

When Gray found that some of the heirlooms had been stolen, she confronted the soldiers and told them not to touch any of “Mrs. Lee’s things,” according to the Park Service.

Gray, whose parents had also been slaves, then complained to Union Gen. Irvin McDowell, and the remaining heirlooms were sent to the Patent Office for safekeeping and posterity.

“She had incredible courage,” Anzelmo-Sarles said. “So we owe a lot to being able to tell the story of our first president to this enslaved woman.”

Gray was freed in December 1862, according to the will of Custis, who ordered that his slaves be freed five years after his death, Anzelmo-Sarles said. He died in 1857.

Gray and her family bought land near Arlington and grew and sold vegetables. She died in 1907.

The Park Service said the photograph will be unveiled to the public at 2 p.m. Saturday as part of a special program on African American history at Arlington House.