Tom Sutcliffe: Can we ever be sure that a picture is telling the whole story?

"Iconic" is the word that's always used – a grossly overused word these days but perhaps justifiable when applied to Robert Capa's famous photograph of a Republican soldier apparently falling in battle. As an object of contemplation, fraught with ideas of mortality and sacrifice, it operated as an icon for many – and it was Capa himself who uttered the words that were to sustain the long and sometimes angry debate about its authenticity. "If your pictures aren't good enough," he said once, "then you're not close enough."

The aesthetic quality of the picture hasn't changed since the day the image was first developed. It works because its proportions open up a poignantly unfilled space to the right of the figure, a place he should be next but will never reach. It works because the downward slope of the hill and the backward fall of the man are in tension and because the shadow of the man and the silhouette of his rifle carve out an angle which echoes his crumpling legs.

But the journalistic quality of the picture has rippled and shifted for years – now a photographer's confection, now nothing less than the truth. And for the most part the defence has rested on the identification of the soldier as Federico Borrell Garcia and the identification of the place as Cerro Muriano, where Garcia was killed on 5 September 1936.

Having relied so heavily on those corroborating facts, Capa's defenders are going to have a serious problem if El Periodico's assertion that the picture was actually taken 30 miles away are irrefutable. But that someone will have a go at refuting it is almost certain. Devotees don't surrender their icons without a fight – and for those convinced that this is an emblem of photography's ability to capture the most fatal of decisive moments, the fight is worth having. For most people now, though, it has become irretrievably an icon of something else altogether: of the uncertainty that we can ever be sure that a picture tells the whole story.