In 1968, when the photographer Don Honeyman was experimenting with Alberto Korda's iconic image of Che Guevara, he discovered something curious. Honeyman had been experimenting with a process of solarisation as a way of making fashion images more exciting and had been asked by a poster company to try the same thing with Korda's photograph of Che - said to be the most reproduced photo in the world. But he was having trouble duplicating the look of the image as it had first been published in Europe by the revolutionary press.

"I worked over the image for several days," Honeyman wrote, "but couldn't seem to get the same idealistic gleam in Che's eyes. I finally compared the first Che with the second, and discovered that some canny designer, presumably at [the original Italian printers], had made Che slimmer and his face longer, by about one-sixth. It was so effective that I, too, stretched him, and it worked like a charm. It doesn't really do to have a revolutionary who's too plump."

There is something fitting about the world's most iconic revolutionary image having been manipulated. Che's legacy, 40 years after his death in a failed attempt to ignite revolution in Bolivia, rests heavily on an image so powerful and so plastic that it still serves both as a generalised inspiration to rebel and as a vehicle for the sale of everything from ashtrays to T-shirts.

The photograph was taken in March 1960 at the funeral of the victims of an explosion on board the French freighter La Coubre in Havana harbour, in which 81 people had died. The Cuban leadership suspected sabotage by the CIA and the funeral, attended by Simone de Beauvoir and Jean-Paul Sartre, among others, became an anti-American rally. Guevara did not speak, and came into view only briefly for Korda, who was recording images of the event from the crowd. Korda had started out as a fashion photographer, but was then Fidel's personal photographer. He managed two shots with his Leica before Guevara disappeared from view.

The pictures were not published in the reports of the event, but Korda pinned them up in his studio in Havana, and in 1967 gave two of them to the Italian publisher Giangiacomo Feltrinelli, who was planning to publish Che's Bolivian Diary. Within six months Che had been assassinated in Bolivia and both Feltrinelli and the Cuban government published the first posters.

Even in death, Che was lucky with his photographers. Freddy Alborta, the only professional photographer allowed to see his body shortly after his execution in Bolivia, wired a haunting photograph of the corpse, lying on a table, surrounded by military men. The image is Christ-like and has been compared both to Andrea Mantegna's Lamentation Over the Dead Christ and to Rembrandt's Anatomy Lecture of Dr Nicolaes Tulp. But it was Korda's lucky shots that ensured Che Guevara was not for gotten. Korda's photograph, suitably doctored, took on a life of its own, creating an irresistible combination of celebrity and rebel glamour that gave Che an influence in a world that had long forgotten the details of his exploits.

Through the image, the complexities of Che's life and thought are reprocessed into an abstraction that can serve any cause. It was later used in a fake Warhol, a fake that Warhol authenticated, on condition that the revenues go to him. Che's transformation from revolutionary martyr to pop celebrity, with all that it implied in ubiquity, was complete. Forty years on, it is still going strong: when the Victoria and Albert Museum in London mounted an exhibition last year of the history of the Korda image, the curators assembled objects from more than 30 countries, used in contexts as diverse as Madonna's album American Life and Ricky Gervais's Politics DVD to Jean-Paul Gaultier's sunglasses campaign. It has been painted as graffiti in Bethlehem, carried in demonstrations from Palestine to Mexico and borrowed by such artists as Pedro Meyer, Vik Muniz, Martin Parr and Annie Leibovitz. It has been used to represent causes as diverse as world trade, anti-Americanism, teenage rebellion and Latin American identity. It has sold dolls, French wine, model cars, cigarette packets, stamps, Swatch watches, Austrian skis, ashtrays, mugs, keyrings and nesting Russian dolls. Nor is it under capitalism only that Che's image stimulates sales: souvenir shops in Cuba are festooned with Che tourist tat, and in Bolivia, where the left-wing president, Evo Morales, has installed Che's image constructed from coca leaves in his presidential suite. Tourist agencies even offer package tours to the spot where he died.

Emotional appeal

Che's durability owes little to his revolutionary achievements, though his revolutionary credentials are authentic. He was radicalised as a young man by the US-backed coup in Guatemala that overthrew the elected government of Jacobo Árbenz and he played a central part in the Cuban revolutionary struggle. After the revolution he served as finance minister, but grew increasingly alienated from the Castro brothers. He went to the Congo to support revolution there before setting out on the fatal Bolivian adventure, hoping to spread revolution across the subcontinent. Ernesto Guevara was certainly a revolutionary, but so were many others whose names have long been forgotten and whose records inspire more critical assessment.

Che's appeal is emotional. His death in Bolivia as a relatively young man created Che as secular Christ, the man who took upon himself the sins of the world and gave his life for the cause of the oppressed. His memory remains available to the oppressed; his image continues to inspire the hope of change and the virtue of rebellion, enhanced rather than diminished by his defeat. Christ, too, was defeated on earth and, again like Christ, Che's death conveys a promise of redemption through inspiration. He is the rock-hero biker revolutionary, the martyr to idealism, a James Dean in fatigues. When Pope John Paul II celebrated mass in Havana's Revolution Square, the giant image of Che that hangs there served as a revolutionary counterpoint.

But beyond his quality of universal icon of rebellion, what survives of Che's life's work? The promotion of Marxism and violent revolution? Forty years after his death, it is hard to imagine what an octogenarian Che would have felt about his younger self or about the world that he did not live to see. Would his personal and political asceticism have survived in an age in which rampant consumerism has captured the mass imagination? Would he have been distressed or gratified that the USSR, embraced by Fidel Castro against his objections, had collapsed? In 1964 he called Russia a "pigsty" because of the conditions in which it kept the workers. Would he have been any more gratified by the conditions of Cuban workers, nearly 50 years after the revolution? Would he have been encouraged by the rise of China, whose revolution he praised, or appalled at China's new character as a state-managed market economy?

In Cuba his image serves the mythology of the revolution that is used to glamorise a sclerotic state structure: old men in freshly laundered fatigues preside over a dollarised economy, heavily dependent on tourism, in which young women turn to prostitution to buy the consumer goods their counterparts in Miami take for granted.

In wider Latin America, his legacy is mixed. The perceived failure of the neoliberal reforms of the 1990s has intensified opposition to the Washington consensus and produced a series of left-wing victories at the ballot box that guarantee his name is honoured - as in 2006, when Daniel Ortega's Sandinista movement, now a party of dubious revolutionary credentials, was elected to power and the party faithful wore Guevara T-shirts to the victory party. Hugo Chávez, the populist leader of Venezuela, who is known for his eagerness to wear the clothes of the Cuban revolution, often dons a Che T-shirt. Some of his ideas, too, are back in vogue with Latin America's new left: pan-Americanism, support for the region's popular movements, nationalisation and centralisation of government. The various "expressions of the popular will" that he favoured over ballot-box democracy - neighbourhood courts and the Committees for the Defence of the Revolution - have found new expression in Venezuela and Bolivia.

But even here, Che might have felt a little unease. He was critical of much of the Latin American left for its rejection of the armed struggle, and grafted his Stalinism on to the tradition of revolutionary petit bourgeois nationalism in Cuba exemplified by José Martí, much as the Sandinistas were to use Sandino as an inspiration in Nicaragua. Yet many of those now most enthusiastic about his memory came to power through the ballot box. Only in Colombia, where he remains an inspirational figure for the dissident Farc, would he recognise true heirs.

Politically, there is no movement that could be called Guevarist. In Peru, Fidelistas and Guevarists are in opposing camps, as they are in Panama and Mexico. For contemporary intellectuals of the left, Che's legacy, with its romanticism and heroisation of the guerrilla, is problematic. For instance, Jorge Castañeda, the Mexican writer and sociologist, wrote in his biography of Che that Che's ideas had nothing to offer present generations. For Castañeda, his "refusal of ambivalence" and his unwillingness to understand life's contradictions were relics of a damaging era in Latin America. In an age in which the absolutes of Marxism and market capitalism were judged to have failed, Che had nothing to say.

Nor has his popularity in the west translated into any coherent politics. Che's image is still carried by the left, but is also adopted by thousands who have only the vaguest idea of his life, beyond the Hollywood version of The Motor cycle Diaries. In London, a small torchlit rally held in Trafalgar Square to commemorate the 35th anniversary of his death gave a flavour of the portmanteau character of Che's image. In the heroic prose of the participants, "banners and placards were held high" and "chants and speeches rang out from the megaphone across Trafalgar Square to the listening ears of the demonstrators and the passing public and readings from Che's writings were read out". Speakers came from Rock Around the Blockade, Fight Racism! Fight Imperialism!, Victory to the Intifada, the Colombia Solidarity Campaign, the Africa Liberation Support Campaign and the people's movement of the Philippines.

To this assorted list, as to oppressed peoples elsewhere, Che has little to offer as a guide to making revolution. What he does have is the messianic image of sacrifice for the sins - or sufferings - of others. Regardless of his failures and contradictions, or the obsolescence of his methods and ideology, the potency of that image, with its symbolic, religious quality, continues to inspire.

As the Portuguese writer José Saramago wrote, in characteristically mystical terms: "Because the photo of Che Guevara was, before the eyes of millions of people, the image of the supreme dignity of the human being. Because Che Guevara is only the other name of what is more just and dignified in the human spirit.

"He represents what sometimes is asleep in us. It represents what we have to wake up to know and to learn to know even ourselves, to add the humble step of each one of us to the common road of all of us."