I recall, I think, that this was the largest photograph of war printed during World War I, looking to be something on the order of 8×16 feet or so. This photo of the photo was printed by Underwood & Underwood, and stamped "British Official Photograph", printed sometime in 1918. It seems that this is an actual battle scene, which was a very uncommon thing among war photographs for the Great War–more so for this group of photographs(News Photo Service) produced by pool photographers for equal distribution among subscribing newspapers and periodicals. Actual combat photos distributed by the News Photo Service agencies were not a high priority, unless they depicted routed or retreating or being-defeated Germans–it was not a popularly-disributed subject, mainly for propagandist purposes. (The original photo is available for purchase here.)
I expect that this photo would've given the view a you-are-there effect, as the main characters in the composition were about life-sized–an attempt of slice-of-life experience. There is a surprising amount of foreground in this picture, though.