The picture you posted was taken at the very meeting between Benedikte & Richard when they fell in love. They both attended the wedding between Karl von Hessen & Yvonne Szapary at Kronberg Castle on April 18, 1966 This pic and another one, dancing cheek-to-cheek during that same evening can be found on pages 32 & 33 of the book "Benedikte, Prinsesse til Danmark' by Mette Bacher
The picture showing a very young Benedikte using the famous ruby parure was taken in 1954 indeed, by Court Photographer Elfelt btw. Another one from the same series can be found in the book 'Prinsesse Benedikte, prinsesse i midten' by Randy Buchwaldt