The photo shows Tubman in her 40s and was discovered in an album once owned by Emily Howland, who lived in Sherwood, N.Y., near Tubman's home in Auburn. The photo is distinctive, historians say, because few photos of the famed abolitionist remain and the ones that do show her in her elder years.

Tubman was born in 1820 into slavery, in Maryland and separated from the rest of her family when she was very young. She was forced to work long, hard hours from the time she was 7. She married a free man but remained a slave. In 1849, she escaped to freedom in Philadelphia.

Fearing for her fellow slaves, she returned to slave territory 19 times to escort them to freedom. She never lost a passenger. At one time, the reward for her capture was $40,000. She survived many attempts on her life and even served as a spy for the Union army during the Civil War. She died in 1913.

The photo is in an album containing 43 other photographs, of abolitionists and politicians. Also among those appearing in the album is John Willis Menard, the first African-American elected to Congress. (Menard won a special election in 1868 and was the first African-American to address the U.S. House of Representatives but was ultimately denied his seat by a House committee.)

New York's Swann Galleries are auctioning the album; Swann expects the album to sell for between $20,000 and $30,000. The auction will take place on March 30.

The sale of the Tubman photo comes just a month before the opening of the visitors centre at Maryland's Harriet Tubman Underground Railroad State Park. Tubman will also replace Andrew Jackson on the front of the US $20 bill in 2020.