Keep in mind that WWI was a war of propaganda for the U.S. Wilson reversed his earlier isolationist policies by promoting the idea that we would "make the world safe for democracy".
We relied heavily on the volunteer spirit of Americans and therefore nearly all posters during WWI were sentimental, patriotic, emotional pleas for help.
I love the idea of an anti-war poster, but I would discuss your plan with your teacher first. While it is a unique approach to an assignment, you may be missing the point of the lesson. I would expect to a WWI poster to promote the war effort, not undermine it.
Most of the WWI Posters that I have seen were an attempt to tug at your loyalty to the country and to support the troops fighting the war. I have seen them that try to get people to support troops by rationing goods or by buying war bonds.
Brett D has an interesting point to make. Anyone can make a poster supporting the war effort, but I can’t think the Korean and Vietnam eras were the first to have war protestors. A protest poster might be more interesting and unique. I’m thinking of the war bonds posters Norman Rockwell did for WWII. What about some twisted version of this type of poster than protests the war.
You could go along the lines of encouraging the women to help the war effort...working in the factories, sacrificing luxury items and donating the money normally spent on those items to support the war effort, really capitalizing on the strength of women who are left behind when the men are off fighting for our way of life and the freedoms to which we are accustomed.
As an anti-war protestor, I might draw a picture of the World War I factories churning out weapons and casualties while the Merchants of Death sit atop giant piles of cash - making the point that the same old robber barons were getting very rich off of a war they now wanted Americans to fight in.
I'd probably be locked up under the Espionage Act, but hey, I'd have Eugene Debs and Charles Schenck to keep me company.
The traditional figure of Uncle Sam would be a good character to start with. Perhaps a map of Europe with German territories designated in the background would be suitable. Be sure to use art of proper WWI era uniforms and weapons if you choose to use them.
Looking at this question from the point of view of someone from America or England, I would draw a poster trying to get people to join the war.
The propaganda posters of the time usually used German atrocities as their major theme. They tried to portray the Germans as a bunch of beasts who would kill (and worse) anyone who they got their hands on. So I would try to draw a poster that depicts the danger the Germans represented to England if they were able to win. The poster might show any of a bunch of things -- destroyed buildings, babies and other civilians killed for no reason, women (this would just be implied) in danger of being raped. All of these were common themes in WWI propaganda posters.
For the US posters tended to center on the submarine warfare. The most vivid one I've seen is of a woman holding a baby to her -- they are at the bottom of the ocean having been sunk by a sub.
I would draw things that represent the values of America and have Germany as attacking those values. I would have posters that have women, men, and families on them along with the American flag and have German soldiers attacking them.