World War One was not a broadly photographed war, compared to later 20th century conflicts. The technology of the time wasn't up to the fast movement of a modern conflict. Photojournalism and the 35mm camera had yet to appear so photographers were restricted, by the closed in nature of trench warfare, about what they could take. Photographers focused on VIP's visiting the front, the treatment of the wounded and captured soldiers, as well as other static/controllable scenes. Large cameras were still the main tool of the photographer at that time and they required time to set up and more time still to take an image. Hardly ideal when trying to photograph an advance across no man's land.
The best images from this period tend to focus in on the men and the aftermath of war. The images of the troops, in the various trenches and dugouts at the front, provide a valuable insight into the men and their living conditions. Likewise, the photographs of the dead show the true cost of the conflict on a generation; the ravaged battlefield landscape, littered with the debris of battle, bodies and swathes of mud, are popular subjects for the WWI photographers. The vast majority of photographs were taken using black and white film, but a number were taken for the French army using the relatively new colour autochrome process, developed back in 1907 by the Lumière brothers.
A number of websites have a good selection of World War One photography :-
Finally i'd like to dedicate this post to
PRIVATE JOHN W MARRINER, 12037, Durham Light Infantry, 2nd Bn.
Died 9th August 1915 at Ypres
PRIVATE GEORGE MARRINER, 25/1406, Northumberland Fusiliers, 19th Bn.
Died 16th April 1917 at The Somme
Both were under 20 years of age. Both have no known grave. Both were great, great, uncles of mine. Gone but not forgotten.