The photograph, taken in May, shows Charles and Camilla in the morning room of their London home, Clarence House.
Many of Camilla's personal friends, family members and even former staff were at the party too along with her ex-husband Andrew Parker-Bowles and her sister Annabel Elliot.
The Prince of Wales and Duchess of Cornwall will return to their duties in Devon and Cornwall later in the week.
Camilla was initially blamed by some for Diana's unhappiness as she complained publicly about Camilla's continuing close relationship with Charles.
In the image, the Duchess stands next to her husband, the Prince of Wales as they wear matching navy blazers.
Family portrait: Testino was also invited to take this photograph for Princess Charlotte's Christening.
Charles, who will celebrate his 70th birthday next year, is meanwhile, in a British wool suit, light blue tie and pocket handkerchief.
Photographer Mario said he was "honoured" to document the royal couple on such an important date. He described the duchess, whose birthday is today, as a "beautiful person".
A spokesman for the couple said: 'The Duchess was photographed with The Prince at home at Clarence House by world renowned photographer Mario Testino who has photographed them both several times before.
The Duke and Duchess of Cambridge and Prince Harry were among guests at a glamorous shindig thrown by Prince Charles at Highgrove, in Glouc.
Royal Central will be providing special coverage of the Duchess of Cornwall's birthday throughout the day.
Chinese dissident Liu Xiaobo's ashes scattered in sea
It might have been the Chinese state that incarcerated him, but he died a prisoner of conscience of the entire modern world. Amnesty International China researcher Patrick Poon said Liu Xia, 56, suffered from depression and heart disease .