Interesting photo. I could not find an exact copy in any collections. Looks like it was taken in 1953, at Buckingham Palace, by Baron Sterling Henry Nahum. This was the closest I could find.
https://www.rct.uk/collection/2081177/hm-queen-elizabeth-ii
Baron (Sterling Henry Nahum, 1906-56) - HM Queen Elizabeth II
Three-quarter length photograph of HM Queen Elizabeth II (b.1926), standing and facing forwards with her head turned slightly to the right and her right hand resting on the back of a chair. HM The Queen is wearing a strapless dress with the Riband and Star of the Order of the Garter, a brooch on the..
https://www.rct.uk/collection/2081177/hm-queen-elizabeth-ii
Found another even closer, although colorized, with the right earring out of focus.
https://www.npg.org.uk/collections/search/portrait/mw203723/Queen-Elizabeth-II?LinkID=mp01454&wPage=2&role=sit&rNo=58
