Photoshop, the ubiquitous photo manipulation program that is 20 years old, is now so popular it's a verb in common usage.

You're browsing the online catalogue for Heine, the German interiors-and-everything-else shop, when a "secretary table" catches your eye. The white one looks hideous, but there's a brown one - so you click the picture to see it in more detail. It looks nice, but there's something unsettling about the picture. The table looks fine, but the chair behind it somehow manages also to have a leg in front of the table. It's interior design, as done by MC Escher.

Except this isn't the fine artwork of Escher - it's lousy gruntwork by someone using Photoshop, the image manipulation program that turns 20 next Friday. The image is just one of a whole stream that have been sent to the Photoshop Disasters blog since it started in March 2008. An eerily unreal, doll-like Ashlee Simpson graced its first post.

Photoshop has, like Google, transcended its origins in the world of computing, and become a verb. But whereas "to Google" is almost always used positively to express usefulness, Photoshopping is almost always a term of abuse: "That picture was Photoshopped" has become a shorthand way of saying it is untrustworthy and misleading (Adobe, the company that sells Photoshop, decries its use as a verb: "It must never be used as a common verb or a noun," it tuts. Too late.)

Examples of its use, or misuse, are legion: a faked image of John Kerry and Jane Fonda apparently sharing a platform at an anti-Vietnam war rally which dragged at Kerry's 2004 US presidential bid; a picture of missiles being fired at Lebanon by an Israeli jet which turned out to have been "tweaked" by the photographer - there was only one original missile; Kate Winslet's legs magically elongating when she appeared on the front cover of the February 2003 edition of GQ.

The defence put up by Dylan Jones, GQ's editor, of the Winslet images was telling. He said that her picture had been manipulated "no more than any other cover star", and that "practically every photo you see in a magazine will have been digitally altered in this way . . . these pictures are not a million miles away from what she really looks like". In other words, that's not actually what she looks like. And, Jones is saying, we should be used to it by now.

Altering images is certainly nothing new. The technique of "retouching" photos and fiddling with negatives has a long and inglorious history dating back to the 1860s, and one stirring picture of General Ulysses S Grant astride a horse in front of his troops at City Point, Virginia, during the American civil war. It turned out to be a composite of three pictures, in which the body isn't Grant's at all.

Stalin's infamous purges also included photographic ones, of all the political figures who had fallen out of his favour. Visual trickery has peppered politics ever since: in 2007, the then UK culture secretary James Purnell was grafted into a picture of the opening of a new hospital.

But it was Photoshop that made altering images routine. It began circumspectly as a program written by Thomas Knoll, who, in the autumn of 1987, was doing in a PhD in computer vision but for fun wrote a program to display images with grey in them on a black-and-white monitor. Knoll called the program Display, writing it on his Mac Plus computer. Then his brother John, who worked at George Lucas's Industrial Light and Magic company, which did the visual effects for the Star Wars films, noticed its potential. They collaborated, bought a Macintosh II - capable of displaying colours! - and set to work; the program's name mutated until they hit on Photoshop.

In September 1988, Adobe Systems signed a licence to distribute it - wisely, the Knolls took a royalties deal that made them very rich. And on 19 February 1990, Photoshop 1.0 became available. At the time it fitted on to a single floppy disk - nowadays it takes a DVD - although it had, even then, fallen foul of piracy after the Knolls demonstrated it to some Apple engineers, who "shared" the demo disks that were left behind with a few hundred of their closest friends. Nowadays, Photoshop is reckoned to be one of the most pirated programs in the world, behind Microsoft's Windows. Its high price - around £560 - is indicative of the fact it has no real rivals.

Photoshop quickly became embedded in computer culture. Apple would try to prove its computers were faster than those running Windows by holding "Photoshop bake-offs" during Steve Jobs's keynote addresses: a Windows machine and an Apple one would run through an automated process to tweak and manipulate an image in exactly the same way. Oddly enough, the Apple machine always won.

Photoshop has even created its own two-player sport, "layer tennis". The first player "serves" an image: the opponent then alters it and sends it back; the first player continues the process. Done in public, with commentary, it takes on its own strange allure.

Do not, though, expect to join the ranks of elite players immediately. Seeing Photoshop running on a computer is like viewing the cockpit of a 747; what, you wonder, do all those buttons do? Many experts say they have taught themselves how to use it over a decade or more. Creative technology consultant Richard Elen describes it as less like flying a plane, more like dealing with a huge house - some people never visit all the rooms. "I probably use 50%-70% of what the apps can do," Elen says. "There are features I seldom, if ever, use. Others I use all the time - clone tools, for instance [which copy an item inside an image] - and I think I'm fairly adept at them."

Russell Quinn, a computer scientist and self-taught Photoshop user, says it's "akin to picking up a guitar for the first time. The whole world is there for the taking, but it's difficult to get started." He thinks two years is a reasonable timescale to get on top of it. © Guardian News & Media 2010

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