We Can Do It!, J. Howard Miller, 1943
In 1943, the american artist J. Howard Miller designed a poster for Westinghouse to motivate the many women who were working during wartime on jobs of men who were in military service. The poster became known under the name Rosie the Riveter and was largely forgotten until a few years ago, but enjoyed a revival in the form of new versions, particularly in the last few months. Read more in Wikipedia.
The poster is what I would call a viral poster, i.e. a poster that spawns many copies which spread like a virus. From observing the number of copies on Pinterest and Google Images, I estimate that currently there are maybe 1000 different variations on internet, with a growth rate of 10 - 20% per year.
Other examples of viral posters are Shepard Fairey's Obama poster, or the british Keep calm and carry on, the russian Ne boltai! ("Don't tattle!") by Nikolai Denisov & Nina Vatolina, the american poster I want you for U.S. army by James Montgomery Flagg, and many more.
The reason why a particular poster becomes viral are, to my knowledge, unknown. I came across them for the first time when working on Deja vue posters, but it seems obvious that the reasons I gave there for multiple variations of a poster have little to do with the phenomenon of viral posters. Plagiarism is definitely not an appropriate concept for them.
Also I guess that many of the "posters" have never been printed and exist and spread exclusively in the internet, particularly on social websites like Pinterest, Flickr, Facebook or Behance.
Below are about 370 variations of the poster "We Can Do It!". High resolution versions and sources are on my Pinterest pinwall Rosie the Riveter.