Prince Harry's girlfriend is said to have posted the photo of herself, showing off her natural beauty and wearing her blonde hair in a bun on top of her head, in order to help raise money for the charity.
Cressida joins stars including Michelle Keegan, Cheryl Cole and Jemima Khan who have helped raise millions to battle the disease by sharing images of themselves without the beauty enhancing products the public are used to seeing them wear.
Cressida's selfie comes just days after she and her royal boyfriend were spotted enjoying a short skiing break in Kazakhstan.
The couple flew by helicopter to the exclusive resort of Shymbulak, a luxury chalet located in the mountains above the city of Almaty.
The holiday will be a welcome break for Harry, 29, and Cressida, 25, who are experienced skiers.
It was on a winter sports holiday in the Swiss resort of Verbier that the couple confirmed they were an item after being seen openly embracing on the snowy slopes.
Harry and Cressida's holiday comes amid increasing speculation that an engagement is on the cards.
In March the couple made their first ever public appearance together, attending the launch of the first WE Day at Wembley Arena, where they sat side-by-side in the stands and Cressida sneaked in a quick kiss on Harry's cheek.
Just two days later they stepped out for another high-profile outing, to cheer on the England rugby team as they took on Wales in the Six Nations.