Why I can't stop looking at the Queen's mesmerising portrait

The Annie Leibowitz portait of Her Majesty Queen Elizabeth II surrounded by her great grandchildren, the newest, Princess Charlotte settled plumply on her lap is, quite simply, stunning. And mesmerising.

The historic setting is perfect in its quiet grandeur, every little figure is beautifully composed and at its beating heart - in art as in national life - sits the Queen. 

Here she is at once unyielding monarch and loving great grandmother; her ramrod back befits a dutiful sovereign yet her softened features with that hint of a smile are those of a woman in old age delighting in the company of the young and the rambunctious.

I can't stop gazing at the picture and part of me really hopes that the original is 30ft across, covering an entire wall of the National Portrait Gallery; as monumental in size as it is in importance.

Because, let us make no mistake, this is an immensely important image that speaks volumes about the Queen past and present and hints, intriguingly, at the future of the monarchy.

Throughout the ages court artists from Holbein to Van Dyck have understood that a picture of the Royal family is about far more than the aesthetics skilfully capturing the penumbra of shadow or the lambert sheen of silk taffeta.

Every object told a story, the globe of conquest, the psalm book of piety, the skull a memento mori reminder that for all the trappings of power, death cannot be evaded.

So too, does Leibovitz's masterpiece artfully convey a wealth of information. That it is a photograph not an oil painting lends it an indisputable modernity yet the Windsor Castle mise-en-scene is entirely traditional; a surprisingly harmonious balance has been struck emphasising the hope, indeed expectation that the future House of Windsor will be able to retain its heritage while expressing its relevance 

The heavy gilt frames that serve as a backdrop are not excessively ostentatious but nor are they particularly finely wrought and certainly far from delicate.

They are solid and fixed (verging, some might say, on the downright oppressive) and subliminally reflect a life lived in the public eye and governed by constraint and ordered boundaries.

And yet. And yet at the forefront of this wonderful study are seven hearty healthy, freshly-scrubbed children - even if Prince Edward's son James, Viscount Severn, hands stuffed into pockets, has the fabulous air of an eight-year-old urchin hauled indoors when he'd be far more comfortable climbing a tree or pond dipping.

But that is a major strand of the picture's charm; no dress code or even colour code has been imposed on its young subjects. The Queen, who adores the outdoors, is a paradigm of informal (as opposed to the altogether sloppier "casual") dressing, and is faultlessly turned out in pearls and checked skirt.

Lady Louise Alice Elizabeth Mary Mountbatten- Windsor (aka James's big sister) meanwhile is wearing just the sort of dress that any 12-year-old would wear to great-grandma's, while Zara Phillips's two-year-old daughter, Mia Tindall is absurdly cute as she holds aloft the Queen's - affectionately known to her as Gan-Gan - £1,000 Laudner handbag.

There's five-year-old Savannah Phillips, elder daughter of Peter Phillips and her three-year-old sister Isla, dressed in pie crust collars and posh cardigans because, one suspects, their mother is Canadian and is trying to keep up with the proverbial Windsors, and wasn’t  aware the dress code no longer demanded photo-Elizabethan ruffs.

But no matter! They look precious as angels flanking Kate and Wills' cherubs.

Two-year-old Prince George, a latter day Christopher Robin but with a more characterful "naughty monkey" streak according to his parents, is scrumptious in his little shorts and knee socks.

His little sister is holding a favoured toy. But it is not a solid silver rattle or an organic wood-carved block. In fact, it appears to be made of (ssssh, whisper it lest the Ravens take flight from the tower) plastic.

And why ever not? This is a portrait with many graces, but its sitters display no airs; in previous reigns children were symbols and pawns millstoned by their aristocratic titles and bred to fulfil their pre- ordained destiny.

These little people are clearly individuals and encouraged to be so. As she presides over them, Elizabeth II looks at ease, relaxed, proud.

Having admired her many gruelling decades of public engagements, her fortitude, diplomacy and devotion to duty vanishingly few of us would want to be Queen in her stead. 

But who among us would not wish, ardently, to cut such a dignified engaged figure at 90, surrounded by the pulsing life and aching sweetness of our very own great grandchildren? 

That is a living, breathing legacy with which anyone, royalist or republican, can identify. Many happy returns, Ma'am.