In one of them, a mother, with her pack at her feet and her baby on her back, sits alone on a railway track. Behind her, the line recedes into a crowded background to give that uncanny air of Renaissance formality that Salgado manages to wrest from every scene of toil or terror. Sunset, woodland shadow and campfire smoke create a rich chiaroscuro, another of his hallmarks.
Yet, as always with Salgado, it's the people who make the picture. Her mouth masked by one hand, the woman gazes straight into the lens with a searing directness. She doesn't plead and she doesn't cower. For his part, Salgado disdains to hide his presence in that phoney, fly-on-the- wall manner beloved of second-rate documentarists. This image is about us, the woman's gaze implies - the sort of idle voyeurs who insist on inspecting the world's misery but not on stopping it, as our own eyes slide over to the ad for "free travel insurance" placed with ghoulish irony nearby.
Because he takes hauntingly beautiful pictures of the people on this earth who sweat and suffer most, Salgado - who started his photographic career as a political exile in Paris in 1973 - often runs up against a knee-jerk liberal charge. He's accused of masking poverty or pain with the anaesthetic gloss of art, and of treating his subjects as passive triggers of pity or curiosity. No one who bothers to look hard at his work could believe that for longer than a shutter's blink. You can trace his mission to explain, and to challenge the spectator, in every detail - from the bold faces that refuse to settle into winsome victim poses to the fussy notes that this trained economist appends to every project.
Terra: struggle of the landless - a joint book-cum-exhibition project which launches this Thursday - embodies the Salgado style in action. Its origins lie in his native Brazil, just outside a village improbably called Eldorado dos Carajas, in the poor northern state of Para. On 17 April last year, unarmed peasants belonging to Brazil's Movement for the Landless (MST) - which represents five million poor rural families - tried to occupy local property in order to cultivate it as self-sufficient smallholdings. Although such takeovers have long been legal in Brazil, land distribution remains as unfair as in 1940 - 75 big ranchers alone own estates the size of Great Britain.
The peasants of Eldorado walked into an ambush by military police, who massacred 19 of them with guns and machetes, and wounded more than 50 others. None of the 155 "police" - some of them hired goons in borrowed uniforms - has yet faced a court.
The 109 images in the book of Terra straddle mythology and economics, art and activism. As it moves from the hunter-gatherer idyll of the Yanomami in Amazonia, through the plebeian wretchedness of shanty-town and plantation, to the raggle-taggle crusades of the MST, it also tells a story about genesis, exodus and ultimate redemption. In his notes, Salgado writes as a hard-headed, practical reformer - he points out that, on one occupied estate, the 500 families farming it will generate a total income four times higher than the cattle-ranching they displaced.
In his photos, though, a Biblical prophet aims the camera (as always, a no-frills Leica with standard Kodak film). Death comes easily and often in these parts, so crosses and coffins - many of them distressingly tiny - punctuate the pictures. Then, characteristically, a blazing shaft of heavenly light will flood over some squatters' camp. Moses and Marx unite in the cry of "Let my people go".
Back in the material world, Salgado has diverted the profits from Terra to fund sets of 50 posters of images from the book. Supporting exhibitors around the world - including nine in Britain - buy these sets for $500 each. The proceeds return directly to MST in Brazil. When it comes to putting his loot where his lens is, Salgado needs no lessons from the prolier-than-thou art theorists.
With luck, Terra should draw its viewers deeper into Salgado's unique world and, in particular, to his epic project "Workers". Completed in 1993, this mammoth "archaeology of the industrial age" cranks up the tension between the glamour of its images and the plight it depicts to an almost unbearable pitch. Mud-caked scurrying hordes of Brazilian gold-miners fill landscapes out of Bosch or Brueghel. Blackened workers amid the noonday dark of blazing oil-fields in post-Gulf War Kuwait slump or strain like bronze statuary. Sulphur carriers tramp over a sublime, but toxic, moonscape in the Java highlands - and so, unforgettably, on, from the ship-breakers of Bangladesh to the Eurotunnellers of Folkestone. The visual counterpart to some sweeping novel by Dickens or Zola, "Workers" brings to light the hidden links between production and consumption. It also marries ethics to aesthetics with a force that few artists in any medium can match.
If you swallow the modish blather which pretends that most people on this planet now earn their crust by flogging junk bonds or designing Web sites, you will probably need hackneyed PoMo stunts involving bisected animals or purloined body-parts to experience risk and awe from art. Stick with the truly taboo notion that the comfort of the rich rests on others' blood and tears, and Salgado remains a giant presence, as a goad and guide. That Javanese sulphur, by the way, will slowly kill its porters - but it may well have preserved the last bottle of wine you bought.
'Terra: struggle of the landless', sponsored by Christian Aid, opens Thursday at St James's Church, Piccadilly, London W1 (0171-734 4511); and at Glasgow Museums and Galleries, and the Universities of Sussex, Cardiff, Warwick, Edinburgh, Oxford, Leeds and Essex. The book is published by Phaidon, price pounds 35