Yellow Submarine art direction and character design by Heinz Edelmann. No dedicated website, unfortunately. XTC (among others) swiped the style for the sleeve of their excellent 1989 album, Oranges & Lemons.
Update: Edelmann is ambivalent about being known as the Yellow Submarine designer but he talked to Baltimore’s 21st Century Radio about working on the film here. And the same site has a look at the film’s production process.