PHOTOGRAPHY / A ride on the back of a horse called Fame: 'Hello] magazine with the neuroses and without the naffness': Andrew Palmer questions the work of Annie Leibovitz, on show at the National Portrait Gallery

War-torn Sarajevo is a real problem. The eight or so images which hang on the concluding wall of the Annie Leibovitz portrait show come at you like a hazardous exclamation mark at the end of an otherwise straightforward piece of prose.

Were it not for Sarajevo, the exhibition would be a neat, two-paragraph affair. First: black and white shots taken mostly for Rolling Stone magazine in the Seventies and early Eighties. Second: high-gloss colour shots taken for Vanity Fair and other magazines and advertising campaigns from the Eighties up to the present day. The former are observational and documentarian; the latter, contrived and interventionist.

Leibovitz's stratospheric reputation rests on her having successfully evolved to meet the requirements of both decade and publication. An avant-garde rock mag processing the liberal ethos of the Seventies would value highly a photographer whose work appeared laid-back, participatory and yet, despite the notorious drug-induced filter, still managed to tell the story. Conversely, the sleek bible of fortune in the Eighties required a photographer who could paint into the images as many self-conscious cues as possible to represent indulgence, ostentation and excess.

That the transition appears so effortless is perhaps because of the continuity of her subject matter. Leibovitz has always had a ride on the back of a horse called Fame. And it is a horse she evidently knows how to control. Be they dignitaries of worlds musical, political, athletic or thespian - all of Leibovitz's subjects seem to display an uncompromising trust in her. The Rolling Stones allowed her to mooch around in the background and picture everything from a comatose, if sexually aroused, Keith Richards to a drunk-as-a-lord Mick Jagger. These photographs betray no less confidence in their maker than do the later, more premeditated shots - for example, Roseanne Barr and Tom Arnold, depicted happy as, er, pigs in mud.

From the works on show it is easy to admire Leibovitz's ability to get close enough to her subject to claim the inside story - her breezy grasp of that most bankable buzzword 'exclusive'. Whatever the seedy connotations of stalking fame, she evidently had the sort of drive (or luck) which got her to the right place at the right time - the White House as Nixon resigned, John Lennon's flat hours before he was assassinated.

The promise of the slickly executed 'exclusive' suffices to account for that oft-quoted six-figure contract given to her by Conde Nast. Superficially, her work for Vanity Fair was a departure from her days on Rolling Stone - colour supplants monochrome, people become active rather than passive adjuncts - but underneath (and this is what Conde Naste was counting on) the message was the same. Leibovitz was the woman to get behind the scenes or beneath the underwear. Were this the whole story of the exhibition we could relax and appreciate a mistress at work, take an entertaining voyeuristic stroll around the navels of the rich and famous - Hello] magazine with the neuroses and without the naffness. We could marvel at her consummate professionalism; smile at her sets; enjoy the delusion of intimacy she has with her subjects.

But then comes Sarajevo. A corpse, a newborn baby, a bloodstained pavement, a kiss on the street, a pair of shoes in the freezer cabinet of a food shop. They are anonymous pictures, and in the context of the constant flow of images depicting the atrocities and hardships suffered in the besieged capital, they are unremarkable pictures - yet, after so much shimmering feelgood, they catch you off guard with their bleating incongruity. It's a question of taste, of course, but the beleaguered city of Sarajevo seems to sit uncomfortably - like a trivialised tribute to the photographer's integrity.

Indeed, the shots appear so disembodied as a series that one can't help but have some sympathy with the malicious rumour that Leibovitz could only manage 11 days in the city before being recalled to Los Angeles for a studio muscle-in with Sylvester Stallone.

Leibovitz has said, on more than one occasion, that so long as she is taking pictures of famous people she will always be criticised. In this light, the inclusion of the Sarajevo photographs goes hand in hand with the exclusion of the two pictures which did more to publicise her name than any others - the shots of Demi Moore before and after giving birth. Given the amount of newspaper space those two Vanity Fair covers commanded it would have been nice to see the prints in the flesh. Why were they omitted? Could it have been that they were too controversial for the conservative gallery-going British public? Hardly: we're talking Leibovitz here, not Robert Mapplethorpe, Andres Serrano or Jock Sturgess.

Quite simply, these images, more than any others, crudely display the anodyne formula which underpins Leibovitz's recent success. When Moore drops her pants, whether pregnant or not, the only taboo to be broken is a Hollywood taboo; we perceive the event as a risk taken by Moore, a risk which may or may not compromise her career, but on second thoughts can only advance it. For, as with so many of Leibovitz's cover shots, the risk is a calculated one, insured by a massive production budget.

That Leibovitz gets people to do what they do is because her world view is so unthreatening. She is a collaborator not a detractor. It is a sweet, self-perpetuating cycle. The more the perceived risk the greater the exposure, and the more recognition for both parties.

Hers may be a novel, even ingenious, way of packaging the stars, but it is nevertheless a packaging fashioned from silver. There are, of course, images which will remain memorable in spite of the fact that their primary purpose was to peddle magazines - for example, that of the heavyweight pugilist Evander Holyfield, whose skin tones are so close in hue to the dark-greys of the flats which box him in, that he seems more spirit than flesh, as if he had been too contemptuous to feature in person.

But for the most part the images are best appreciated in the context of populist journalism: Sting caked in desert mud; Whoopi Goldberg in a bath of milk; a cigar-smoking Arnold Schwarzenegger astride a white stallion; John Cleese mimicking a bat; Bette Midler in a bed of red roses. And while we're at it, most of these made more sense crowned with their magazine straplines than they do here without them.

In case you are wondering why an institution as eminent as the National Portrait Gallery is hosting such a show, a simple answer was given by Charles Saumarez Smith, director of the National Portrait Gallery, at the exhibition opening: 'We cannot have a show without sponsorship; all the Government pays for are the salaries and the buildings.' Annie Leibovitz, it turns out, is here on a PR freebie - courtesy of American Express.

'Annie Leibovitz: Portraits' to 30 May; NPG, London WC2