In 1942, in Nazi-occupied Warsaw, a Polish postal official working for the resistance opened a letter sent by a German soldier to his family.

Inside was a photograph which so shocked the man that he forwarded it to the Polish underground; thus it fell into the hands of a brave 16-year-old called Jerzy Tomaszewski, one of whose tasks was to pass on evidence of German atrocities to London so that the Allies could publicise Nazi cruelties in Eastern Europe.

Tomaszewski made a duplicate of the photograph for London and kept the original. He is still alive and, more than 60 years later, allowed freelance documentary photographer and writer Janina Struk to see the precious and terrible evidence – from which she made a perfect copy.

I will let Struk describe the photograph in her own words as they appear in her terrifying new book Private Pictures – about the private photographs taken by soldiers, from the Boer and 1914-18 wars through to the post-2003 US invasion of Iraq. "Somewhere near the small town of Ivangorod in Ukraine," she writes, "a German soldier points his weapon at a woman with a child in her arms. She has turned away from the soldier and wrapped herself around the child. Her foot is lifted from the ground as though she might be moving away from the soldier or perhaps the shutter has caught the moment the bullet has hit her.

"On the left of the frame are the tips of what look like two other guns pointing in her direction and on the right there appear to be three or four people crouching beside an indistinguishable object. The body of another person lies at the feet of the soldier. On the back of the photograph handwritten in German is 'Ukraine 1942, Jewish Action (Operation), Ivangorod'."

The photograph was to become one of the most impressive and persuasive images of the Nazi Holocaust, although its history would be smothered in the kind of "controversy" Holocaust deniers cultivate. In most publications, the picture would be cropped to show only the woman and the soldier pointing his rifle in her direction, thus giving it an artistic reverence while destroying the context of its original form. The Independent today prints the full width.

In her book, Struk asks why soldiers take pictures of their own cruelty. There are countless authenticated pictures of German troops grinning as they stand next to hostages who have been hanged, crowding round mass graves to watch the execution of Jews, Soviet commissars, hostages, men and women. But I have been studying this particular Ivangorod picture for hours this week. Somehow I can imagine the terrible, excited conversation. "Hey, Hans! On your left, they are shooting Jews. Get your camera. Look at that woman run!" SNAP. Or was the cameraman an off-duty killer? We shall probably never know. But the tradition, of course, continues. Look at the videos Americans took of their murder victims in Iraq. I shall return to this subject next week.

I enlarged the 1942 photograph to the highest definition and went over it carefully. Then I called Struk. Surely, I said, the "other person" lying at the feet of the soldier is also a woman. She appears to have a parting in her hair, her arms have fallen to the ground on her right and she is wearing a skirt, on the edge of which you can see her left leg. Struk had already spotted this. And then, I said, surely there are four men in all, three in cloth caps and jackets, and the fourth – who appears bigger because he may be wearing a greatcoat. (There appears to be a deep pocket on its right side.)

There is nothing ghoulish in such a study. The more you find in these images, the more you discover and the more real becomes the Holocaust. It may be – look at the picture carefully – that the soldier is actually shooting at the four men and that one of the other two rifle barrels is firing at the woman with the child. The shadows on the ground to the left suggest there may have been many more killers shooting at that moment. But what struck me was the nature of the earth on the right of the picture.

Struk describes an "indistinguishable object" on the right. It appears to be a wooden stake. To the right of it, I see some disturbed earth. Was the stake a marker? "You will dig your own mass grave, up to this point." Is that what the Germans ordered their victims to do? But then – reader, observe carefully – I discovered what is clearly a metal shovel, upside down, its shaft behind the stake. It is identical to other shovels in other execution pictures I have seen. Had the four men been digging their own graves?

Incredibly, when the photograph was used in a book published by the Soviet-installed Polish communist regime after the war, a right-wing West German newspaper, Deutsche Soldaten Zeitung, ran a headline above it "Achtung Fälschung" (beware falsification). The man pointing the rifle towards the young woman and her child was not wearing German uniform or using a German rifle, the paper said. A certain Professor Otto Croy accused the Poles of fabricating the photograph for propaganda.

Then, mercifully, up popped a former member of Hitler's Einsatzgruppen, the "special action" squads used to murder a million Jews in Ukraine. The soldier in the picture is wearing German Einsatzgruppen uniform, he said, and holding the usual Einsatzgruppen rifle. What more proof do you need? Years later, an exhibition of German atrocity photographs in Eastern Europe was put on in Dresden where an old man stared at the pictures for a long time. Then he began to cry. And as he rushed from the exhibition hall, he shouted: "It's me...It's me."