The "We Can Do It!" was actually an American wartime propaganda (WWII, to be precise) poster produced in 1943 by J. Howard Miller for the Westinghouse Electric company as an inspirational image to improve worker morale. It was not initially intended just for women - they had another 41 posters featuring mostly men, work-people and managers. It is generally thought that this particular poster was based on a black-and-white service photograph of Geraldine Hoff, a factory worker.
However, this poster was seen very little during that period (it was only used in the factory for two weeks, more or less). But it was rediscovered in the early 1980's and, then, widely reproduced in many ways, to promote feminism and other political issues. It is often called "We Can Do It!" or "Rosie the Riveter" poster, the last after the iconic figure of a strong female war production worker.
After this period of rediscovery, observers often assumed that the image was always used as a call to inspire women workers to join the war effort. That was not the case. During the war, the image was strictly internal to Westinghouse, displayed only during February 1943, and was not for recruitment but to exhort already-hired women to work harder. Nowadays, feminists and others have seized upon the uplifting attitude and apparent message to remake the image into many different forms, including self empowerment, campaign promotion, advertising, and parodies.