The image of the then-Diana Spencer and her close friend Adam Russell was taken in the late 1970s while they were on a ski holiday together, and it was obtained by bosses at the Daily Mirror in February 1981 - just two days after the tragic royal became engaged to Charles, Prince of Wales.
Editors subsequently chose not to print the picture and instead labelled it "not to be published", deciding to keep it hidden in the paper's archives.
The never-before-seen photo was purchased by executives at the private Caren Archive firm in 2006 and made public earlier this month as it was placed up for sale at the RR Auction house in New Hampshire.
The picture has since sold for thousands after being snapped up by an anonymous real estate investor from San Diego, California.
Diana, mother to Prince William and Prince Harry, wed Prince Charles in July 1981. They divorced in 1996 and she died in a horrific car crash in Paris, France in 1997.