To most she is the Queen, but to her great-grandson and future king, Prince George, she is apparently “Gan Gan”. To mark her 90th birthday, it is in the latter role that the celebrity photographer Annie Leibovitz has captured Britain’s first nonagenarian monarch, surrounded by her five great-grandchildren and two youngest grandchildren.

In a highly stylised portrait, released by Buckingham Palace, it is the woman who is celebrated, rather than the sovereign. With Princess Charlotte, the 11-month-old daughter of the Duke and Duchess of Cambridge on her lap, the Queen is flanked by Prince George, and Zara Phillip’s daughter Mia, both two, Peter Phillips’s daughters Savannah, five, and Isla, three, and the Earl and Countess of Wessex’s children, James, Viscount Severn, eight, and Lady Louise Windsor, 12.

The last time the Queen was photographed by Leibovitz, “Crowngate” exploded, when a trailer for a BBC documentary featuring behind-the-scenes footage of the shoot was edited in such a way as to erroneously suggest the Queen had stormed off in a huff.

Two other official pictures are released today, one showing the Queen on the stone terrace of Windsor Castle with her four dogs, corgis Willow and Holly, and dorgis Vulcan and Candy, and another of her sitting with her only daughter, the Princess Royal.

On the eve of her birthday, the Queen was in Windsor to mark a significant milestone in the history of another British institution.

The Queen poses in the private grounds of Windsor Castle with four of her dogs Willow (top right), Vulcan (top left), Candy (bottom right) and Holly (bottom left). Photograph: Annie Leibovitz/Getty Images

Five hundred years after Henry VIII knighted the first master of the posts, leading to the creation of the Royal Mail as we know it, she visited her local Windsor postal department which was being renamed in her honour.

On being told the Royal Mail delivers over a billion parcels a year, the Queen drily noted: “I’ve probably added to that this week.”

Moya Greene, Royal Mail chief executive, joked: “I have it on good authority that your own postmen and women will be especially busy with tomorrow’s mailbag.”

Thursday’s celebrations are planned to be relatively low key. After a walkabout on her Windsor doorstep, she will later light the first of a chain of more than 1,000 beacons across the UK and overseas before attending a birthday dinner hosted by Prince Charles and attended by members of her family.

Being the Queen she is blessed with two birthdays each year, one actual and one official. More formal celebrations, including a service of thanksgiving at St Paul’s Cathedral and giant street party in the Mall, are planned for her official birthday in June.

In this official photograph, Queen Elizabeth II is pictured with her daughter, princess Anne, in the White Drawing Room at Windsor Castle on 20 April 2016. Photograph: Annie Leibovitz/Getty Images

The Duke of Cambridge, Prince William, paid tribute to his grandmother in TV interviews and spoke of the support she has given him as he described his own sense of destiny and duty.

Of the recent “workshy” jibes, he said he was “going to get plenty of criticism over my lifetime, and it’s something that I don’t completely ignore, but it’s not something I take completely to heart”.

“When the Queen decides that she’s going to hand down more responsibilities, I’ll be the first person to accept them,” he told the BBC. “I take duty very seriously. I take my responsibilities very seriously. But it’s about finding your own way at the right time, and if you’re not careful duty can weigh you down at a very early age.”

His grandmother and father were both very active and engaged, and his family was “extremely supportive” of him not carrying out more royal engagements and having “time and space to explore another means of doing a worthwhile job,” he said.

Thoughts on what kind of king he wanted to be “very much occupies my thinking space”, he said. But it was not a priority at present. It could be twenty or more years hence, he said. “I have no idea when that’s going to be, and I certainly don’t lie awake waiting or hoping for it, because it sadly means that my family have moved on, and I don’t want that.”