"This is so cute, thanks for sharing it with us," one commented, while another added: "Lovely precious memories."
The family portrait showed the monarch and her late husband, the Duke of Edinburgh, sitting down and surrounded by their great-grandchildren, with Prince George at the front and Princess Charlotte waving cheekily from the sofa in-between her great-grandparents.
As some commenters couldn't help noticing, however, one of their great-grandchildren was notably absent: Prince Harry and Meghan Markle's son, Archie Harrison Mountbatten-Windsor.
WATCH: Prince Philip dotes on young Princess Anne in sweet unearthed clip
There's a simple reason for that, though: the photo was taken before he was born!
Although it wasn't shared with the public until Wednesday, it was taken back in 2018.
Royal fans were thrilled by the previously unseen photo
As it was snapped at Balmoral, where the Queen always spends her summers, the Duke and Duchess of Sussex would have been newlyweds at the time.
The royal couple tied the knot in a lavish ceremony at St George's Chapel, Windsor on 19 May 2018 and then went on to welcome Archie almost a year later – on 6 May 2019.
Meghan is now pregnant again, with a baby girl, and has been advised not to fly, so cannot attend Prince Philip's funeral on Saturday.
The royal couple are doting parents
Harry, however, arrived in the country at the weekend and is quarantining before the event.
On Monday, he issued a heart-warming tribute to his late grandfather, in which he light-heartedly summed the Duke of Edinburgh up as: "Master of the barbecue, legend of banter, and cheeky right till the end."
Harry then movingly thanked his "Grandpa" for "your service, your dedication to Granny, and for always being yourself", and said he and his family - wife Meghan Markle, son Archie and their unborn daughter - would always hold a "special place for you in our hearts".
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity, royal and lifestyle news delivered directly to your inbox.