Celebrities are photographed in all sorts of situations, formal and informal. And I believe that particular photo was one taken by Diana's father Earl Spencer when she was a young teenager, long before she ever dated Charles.
This photo of a young Princess Diana reminds me of a celebrity.
It was taken in 1990 by Sir Terence Donovan.
Princess Diana wearing a Bruce Oldfield dress
http://www.alamy.com/princess-diana-wearing-a-bruce-oldfield-dress-33273690.html
A Turkish artist used AI to create portraits of famous people who died young how they would look now. Amongst them Diana.
https://www.instagram.com/p/Ci3ldsSvcKY/?igshid=MDJmNzVkMjY=
https://www.thenationalnews.com/ar...artist-uses-ai-to-reimagine-dead-celebrities/