It's also worth noting that it was only one of three posters made, the other two just weren't popular at all.

The wording of ‘Your Courage… will bring us victory’ was criticised. There was some evidence the combination of ‘your’ and ‘us’ ‘suggested to many people that they were being encouraged to work for someone else’, with the ‘your’ referring to the civilian, the ‘us’ to the Government.
[...]
‘Freedom is in Peril’ was also deemed ineffective, blamed on ‘the abstractness of the words, not one of which had any popular appeal’.[55] Even during the planning stages the criticism had been raised that ‘Freedom’ was rather an abstract concept and was ‘likely to be too academic and too alien to the British habit of thought’.

• https://web.archive.org/web/20150402071239/http://ww2poster.co.uk/2009/04/1939-3-posters/

Reconstructed examples of the two other posters, "Your Courage" and "Freedom is in Peril" are seen here:
https://en.wikipedia.org/wiki/Keep_Calm_and_Carry_On#Design