Posters have been used for a long time to convey messages to the public, they can be a very effective way in my opinion to evoke a feeling in someone by either displaying a message or an advertisement. There are many famous posters from American history that really stick out in my mind as effective and outstanding such as the famous Uncle Sam "We Want You" poster.
One poster that I always liked was J. Howard Miller's, "WE CAN DO IT!" design from the 1940's. This was a poster made during the absence of a lot of the men in the war, to boost the morale of the women and empower them in the workplace. At this point in American history women did not have the same rights as they did today, this poster was a voice for the women to speak up and say, "hey we can do this!"
This poster is often times called "Rosie the Riveter" who was actually an strong iconic female production worker of the time. One interesting fact about this poster was that during World War II, this poster was no where near as famous as it is today. The poster made a come back in the 1980's when people started using the poster to promote feminism, and other woman's rights issues. This is the beauty of this poster, which at the time was aimed to encourage already heard working women to continue working, and now is used all over the world to promote women in all senses. The bright colors of the poster drag in your attention today as much as I am sure it did in the 40's, the woman in the picture shows strength and confidence, with possibly grungy clothing, yet a pretty made-up face.