A visual metonym is a symbolic image that is used to make reference to something with a more literal meaning. By way of association the viewer makes a connection between the image and the intended subject.
This poster uses the visual metonym of the crown of King George VI, with the words 'Keep Calm And Carry On' as a way of portraying a message from the King to his people that all capable measures to defend the country were being taken during World War II. In 1939 the British Government's Ministry of Infromation were commissioning propaganda posters to be displayed in the UK. However, this poster was almost unseen by the public as it wasn't officially issued, and was found 50 years later. It uses a white image and type to make them stand out from the bold red background which signifies importance for people to read. The crown symbolises royalty, and the viewer can make a connection between that and the intended subject, which is the message from the King.