Political photography: The illusionists

What is the power of images? Take this one. It shows Tony Blair, in shirt and tie, facing more or less front, and grinning hard, as he takes his own photo. But behind him the scene is entirely filled with the smoke and fire of a massive explosion, blowing the desert apart. Its detonation seems to be simultaneous with Blair's snap.

Obviously it's a composite image. It's entitled Photo-Op and it was made a couple of years ago by our leading exponent of photomontage, Peter Kennard. The figure of a self-snapping Blair was extracted from a news photo (originally, what he had behind him was a group of children and naval cadets). It's then been superimposed on a bit of shock-and-awe from Iraq.

The picture is a great coup. It catches Blair at his most Blairite – the casually contemporary guy, the publicity narcissist, in full grimace. And the whole scene is very nearly believable. It's the kind of thing that Blair, in his boyish gung-ho silliness, could almost have done – gleefully snap himself, with his own war as a souvenir-backdrop. The composite is so seamlessly realistic that the eye can't unpick it. This is no cut-and-paste collage. Photo-Op is made on Photoshop.

So what's the power of this image? Well, it delivers a brilliant insult, in the way that it manages to condense a lot of suspicions about Blair's character and priorities. And it plays a neat pictorial practical joke (which even a viewer who didn't share those suspicions might enjoy). But for all that it's morally apposite and visually clinching, Photo-Op doesn't actually prove anything. For what can a montage prove?

Kennard has been taking images apart and fitting them together for almost 40 years. If you go to his retrospective at the Pump House Gallery in Battersea Park, you can see work going back to the early 1970s. There are attacks on the government of Edward Heath, illustrations for the Workers Press, old bits of artwork coming slightly unstuck. The causes may be fading history, but Kennard's grim wit and graphic impact are already clear.

Then there's his best-known work, the imagery he made for the GLC and CND in the 1980s, including the original collage – it's in colour, though much reproduced in monochrome – for the one with nuclear missiles loaded onto Constable's Haywain. And we come up to date with his recent image-campaign against the Iraq war, now made on computer, and in collaboration with Cat Picton Phillipps.

What photomontage has over a cartoon is that it isn't just an external commentary on the state of the world. It lays hands on a bit of the world – and pins it down, implicates it, exposes it, in a new image. In other words, photomontage depends on a kind of image magic, a 2D voodoo, in which what is done to a photo of something is (we feel) done to the thing itself.

How it is done, technically, makes a difference. Kennard used to believe in leaving his cut edges conspicuous. And certainly, the visible grafts and sutures of cut-and-paste have a special force. They stress the violence of collage's interventions, and the clash or strain between one image and another, eg nuclear weapons vs traditional rural idyll.

The smooth image-fusion of Photoshop has a different tendency. It can achieve the indelibly compromising realism of a blackmail photo, as in Photo-Op. It makes the conjunction of one thing and another visually undeniable. It's good at insinuating guilt by association, while raw collage is better at conveying outrageous, incongruous wrongness.

But it's only a shift of emphasis. Pure collage can be compromising too, and Photoshop doesn't totally lose that sense of clash. The point of montage is that its image-splices do both – clicking together and splitting apart, before your eyes. And whatever the technology, scissor or computer, there's always a good deal of luck involved in finding the images that will work, and work on a number of levels.

Among the recent pieces, Photo-Op is a bull's-eye. There's another good one that has three clock faces showing the times in New York, London and Baghdad (as in a travel agent), but in the Baghdad one the clock's hands are the blades of a hovering military helicopter. It is a sharp visual likeness, and hands/blades both rotate, and the implication of "round the clock" occupation follows.

On the other hand, the one that shows the Israeli "Separation Wall" being battered down by a wrecker's ball, except that the ball is in fact a huge, shiny red Christmas tree ball – I simply don't get it. A visual pun links ball and ball, and I suppose there's some association via Bethlehem and Christmas. But a Christmas tree decoration is the last thing to use on reinforced concrete. Or are we meant to think of powerless goodness miraculously destroying oppressive power?

But the one that implies a link between the green of the dollar bill, and the horrible gassy green tinge of infrared night-battle photography, is interesting. It hasn't really found that link, in a clinching visual way, but it's on to something. It wants you to feel that war is a kind of world-infecting gas, emanating from the dollar.

Of course that sounds a bit mad when you put it into words, but it is the kind of associative manner in which images often "argue" (Titian's as much as Kennard's). Which may be a good reason for not putting too much trust in images, even when you feel they're on your side.

Kennard's work has always been broadly lefty, but pretty consistently negative in its approach. He attacks. He criticises. He exposes. He has never depicted an imaginary better world, or glorified any actually existing alternative. He certainly had no time for the Soviet Union.

Still, any background history to Kennard would need to mention, not only John Heartfield's anti-Nazi montages (an obvious inspiration), but also the work of the Soviet photo-artist Alexander Rodchenko. Whatever his specific allegiances, Rodchenko is one of the founders of modern visual activism. And if you go to the Kennard show, you should see the Rodchenko exhibition at the Hayward Gallery too.

It covers a short span, mid-1920s to mid-1930s: roughly the period from Rodchenko's rejection of traditional painting as "bourgeois subjectivity", to the authorities' condemnation of Rodchenko's photography as "bourgeois formalism". But in those years, armed with a confident sense of living in a new world, and acquiring the new, highly portable Leica camera, the artist created – in photos and photo-montages – a visual language for the Russian Revolution.

His stance is all-celebratory. Obviously he doesn't attack the revolution. He doesn't attack its enemies, either. There are no references to exploitative capitalists, obscurantist priests, militarist generals, White Russian insurgents, or any of the usual roster of Soviet villains so vividly illustrated in Eisenstein's early films.

Rodchenko's energies are positive, undestructive. He shoots tall buildings, people in motion, machinery, vehicles, heroic young pioneers. His trademarks are the view up from below and the view down from above, the vertiginously titled angle – the "Rodchenko angle" it got called – the extreme close-up or extreme long-shot. This is not just a world turned upside down. It's a world where it's hard to tell what is up or down, to get your bearings in at all.

So, though celebratory, it's a risky kind of celebration. Full of unstable slants and disorienting perspectives, it revels in a world in the throes of change. It's not a visual idiom that can easily express progress, or concerted struggle, or consolidation. It's tied to the very moment of revolution when suddenly everything seems possible. And you don't have to be a Stalinist to see that this could never be the official imagery of a state that was trying to settle down.

You do perhaps need to be a Stalinist to feel that it is dangerous enough to need banning. But actually many of us have a rather superstitious belief in the power of images, for good and for ill, or we do when we talk about them. But Rodchenko's dizzying angles never made anyone fall over. And Kennard's incisive splices never forced anyone to a conclusion. On balance, that's probably just as well.

Peter Kennard – Uncertified Documents, Pump House Gallery, London SW11 (020-7350 0523), to 30 March; Alexander Rodchenko – Revolution in Photography, Hayward Gallery, London SE1 (0871 663 2519), to 27 April