The picture, which features the queen with a slight smile as she looks past the camera, was taken at her request around her 80th birthday by renowned photographer Jane Brown, who died in 2014.
“Brown (a fellow octogenarian) has worked as a professional photographer since the late 1940s," the Royal Collection Trust writes in a description on its website. She had been a principal contributor to the British newspaper the Observer, according to the site, since December 1949.
London's National Portrait Gallery acquired some of Brown's photographs, and in 1995, she was appointed a Commander of the British Empire for her contribution to photography.
"Brown is known for her unpretentious technique," according to the Royal Collection Trust, "working at speed, using only available light, and for working in black and white rather than color.”
“Taken last month in the grounds of Windsor Castle, the photo is the fourth in a series of photos released by the Royal Windsor Horse Show, following images that celebrated the Golden Jubilee, the Platinum Jubilee and Her Majesty’s 90th Birthday,” the royal family captioned the photo on its Instagram page.
The queen’s death means her eldest son, who has been the longest-serving heir to the throne in British history, is now King Charles III.
The new king grieved his mother's death shortly after she died.
“The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family,” Charles, 73, said in a statement.
“We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother. I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world.
“During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which The Queen was so widely held.”