FirstWorldWar.com has a deep archive of WWI pictures, which are clearly labeled and sorted. Whether you are doing a historical fantasy, steampunk, or staging a war in your science-fictional world, this will prove fascinating reading.
Besides the photos, there’s also a collection of primary documents that document the course of the war via source – often official government – material.
Some of the archive documents were asigned in the late 1830s and bore relevance to the outbreak of war some 75 years later. There are memos, letters, treaties and the text of speeches throughout the war and beyond.
Thanks to Daniel Poeira for the link