Does 'illustration' mean a picture used for explaining sth?
In the early 20th century, the poster continued to play a large communication role and to go through a range of styles. By the 1950s, however, it had begun to share the spotlight with other media, mainly radio and print. By this time, most posters were printed using the mass production technique of photo offset, which resulted in the familiar dot pattern seen in newspapers and magazines. In addition, the use of photography in posters, begun in Russia in the twenties, started to become as common as illustration.