Also, propaganda posters were created at a time when the general public had no direct access to real-time information. All you learned about was from the newspapers or the radio. The power of a large, colorful image with a short slogan was therefore very strong. Today we are bombarded with large color images, lots of advertising, and we know that what we hear ought to be taken with a pinch of salt. Also at that time there was blind trust in our leaders, which is no longer the case... and as you said, the internet has played an crucial part in changing this (for better and for worse, because it is also responsible for a lot of nonsense and rumors).