The Atlantic recently published an excellent introduction to the causes and conduct of WWI, with an excellent array of photos depicting the machinery and humanity of the conflict. It's a potent reminder of trench conditions as well as the mutual respect shown by the combatants. See picture # 23, depicting a German motorcycle messenger reading a grave marker that says "Hier ruhen tapfere franzosische Krieger," which translates as "Here rest brave French warriors."

http://www.theatlantic.com/static/infocus/wwi/introduction/

Here are a few more interesting photos, showing airplanes in formation as seen from one of them and Indian troops in a gas mask drill.