These early pinups were often marketed as "French postcards," and they were popular among soldiers and sailors who were looking for a bit of fantasy and escapism during their time away from home. The pinup style really took off in the 1940s, during World War II.
As an image to be literally pinned up (on a wall or locker door etc.), the pin-up was born in the United States in the years 1920 1930, during the golden age of the press.