"Putting up or changing door gods (门神画) is an important custom among the Chinese during Spring (春节 New Year) Festival. Door gods are pictures of deities posted on the door outside and inside the house. They are expected to keep ghosts away, protect the family and bring peace and good fortune. (...) A typical Chinese house has a huge front gate/door with two wings that open in the middle. The door gods always come in pairs facing each other. It is considered bad luck to place the figures back-to-back."
Formally, there was a distinction in the use of these protective images. Military themed ones (武) were intended to be put up on doors facing the street. Literary, or civilian ones (文) were to be used indoors.
In modern times, the traditional gods have been replaced by more updated images: peasants, workers, soldiers, children, etc. All are intended to integrate the protective aspects of the traditional door gods with more forward looking elements. As these images continue to be published, we must assume they still strike a chord with the population.