Potentially Useful Subject Headings
Oxford Art Online (above) May have biographical information on the graphic artists responsible for the posters.
Newspaper & magazine articles from the time of WWI are useful for getting a feel for what the war was actually like for participants and for those on the Home Front. When searching the full-text databases of newspapers & magazines below, you will probably want to limit your search by date to the relevant time period (eg., 1914-1918). When searching you will need to think of synonymous keyword search terms for your topic.
Also, at the time of the conflict, "World War I" is not used, of course. "World War," "World's War" & "Great War" tend to be used in reporting.
American Periodicals Series (above) now includes American periodicals through about 1930 & provides coverage for WWI.