Ernest Wallcousins was a renowned and successful painter and illustrator famed for his portraits of Sir Henry Wood, the conductor of the Proms for over 50 years, and for that of Sir Winston Churchill. Wallcousins worked across a wide range of medium and subjects; a book illustrator in the early years of the 20th Century, he designed posters in the 1920’s, including one for the London Underground in 1925 and was renowned for his illustrations of Royal occasions (Macconal-Mason), whilst Wikipedia has limited information about his work as an illustrator.
The photo is provided under Creative Commons Licence from The National Portrait Gallery (NPG x24399).
Keep Calm and Carry On Artist?
Almost certainly the designer for the first three posters for the Ministry of Information, including Keep Calm and Carry On, see TNA INF 1/720, p69, minutes from a meeting on 18.5.39:
Outdoor Publicity: The sub-committee had established contact with Mr Wall-Cousins who had produced some preliminary poster roughs which might, however, be more profitably considered by the meeting next week, when they would be in a more finished condition.
In June, he was still designing poster-roughs, which were submitted for inspection by 8th June 1939, discussions and designs continued, with Surrey Dane suggesting ‘Keep calm. Do not panic’ the following week, and poster designs signed off in early August.