The image, which shows Charles and Camilla in high spirits and clapping proudly, is inside the card next to the festive greeting "Wishing you a very Happy Christmas and New Year".
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Eagle-eyed royal fans have noted that there is another prince in the photograph, Crown Prince Frederik of Denmark, who is seated behind Camilla.
Also visible in the picture are the couple's tributes to the armed forces.
Camilla is pictured sporting a brooch given to her by the 4th Battalion the Rifles, of which she is Royal Colonel, on the lapel of her Bruce Oldfield coat, while Charles is wearing an Armed Forces Veterans Badge and a pin for the mental health charity Combat Stress.
The Prince of Wales, 66, and the Duchess of Cornwall, 67, showed their support for The Invictus Games at its opening ceremony on 10 September.
Charle and Camilla stepped out London's Queen Elizabeth Olympic Park with Prince William, though while the Duchess of Cambridge, née Kate Middleton, had hoped to join them she was still suffering from Hyperemesis Gravidarum.
On 11 September, Charles was back at the Games to enjoy the athletics with his two sons and meet with medal-winners.
The trio were spotted laughing and joking with one another throughout the day as they watched numerous sporting events and spoke to the inspiring athletes taking part.
Charles' pride in his 30-year-old son is no wonder, given that The Games were a huge success. Harry, who said he "would love to keep it [the Games] in the UK," has said he plans to organise it all again in 2016.