Uniforms and (particularly) equipment changed a great deal during The Great War and added to this are the variations for different climates in theatres such as Africa and the Middle East.
BEF troops in 1914
Early War Western Front
Early colour photograph of French Troops in Horizon Bleu
Mid War Western Front
This covers the period of the first trench lines going up December 1914 to January 1916. British wearing the softer cap with flaps, the French moving to Horizon Bleu and the Adrian Helmet. The Germans removing the spikes on their helmets. The need for gasmasks.
German soldier at Verdun 1916
Late War Western Front