National Geographic has a gallery of vintage national park posters from the 1930s.
Created in the mid-1930s in response to the Great Depression, the Works Progress Administration and its Federal Arts Project were focused in part on providing artwork for public buildings while assisting struggling artists. Artists were tasked with creating posters that promoted the landscapes and wildlife of America's parks. The program ended in 1943, and the largest collection of WPA-era prints–including the selection in this gallery–is now in the Library of Congress.
Photo gallery: Vintage U.S. Park Posters [National Geographic]