With regards to the 100th anniversary, the German daily Süddeutsche Zeitung has published an interesting collection of high-quality photos, organized in chapters like Western Front, War at Sea, etc.
http://www.sz-photo.de/suddeutsche-zeitung-photo/kalender/erster-weltkrieg/
Examples:
Belgian troops
http://bilder.sz-photo.de/sz/index.php?offer_id=890096
Tsarist troops in France
http://bilder.sz-photo.de/sz/index.php?offer_id=890099