This term is applied when a part is used to represent the whole, or vice versa. When the main subject is substituted for something that is inherently connected to it. This substitution only works if what the synecdoche represents is universally recognized and understood, rather than taken at face value for its literal meaning.
Within this piece, it uses the visual synechdoche of an image of Lord Kitchener as a substitution within the tagline 'Britons [Lord Kitchener] want you'. It works because he was easily recogniseable to the public as the Secretary of State for War, and the way he is pointing his finger directly at the viewer, it immediately drags you in, making this the most famous image used in the British Army recruitment campaign of World War I.