A few WWI photos and posters I've found, mostly on Retonaut.
ANZACS at the Pyramids.
British Soldiers inspect an observation "tree"

A French anti-aircraft machine gun.

British soldiers before a game of gasmask football.

RFC gunnery training.

Men of the 369th Infantry Regiment "The Harlem Hellfighters" while attached to the French 16th Division.

An advertisement for Albatross.
Fragments from France cover.

"The Better 'ole", I watched the silent film by the same name made in 1926 (it is hilarious btw).

Another advertisement poster.

A RN recruitment poster featuring Nelson's flagship HMS Victory.

A recruitment poster for the US Army Air Service.