This paper presents an application of Relevance Theory (Sperber and Wilson, 1995) to pictures by studying the role that weak implicatures may play in the persuasiveness of multimodal argumentative discourse. We take a relevance-theoretic approach to the discussion of visual and multimodal argumentation with a particular focus on the role of onomatopoeia. To examine the possible mechanism by which persuasion operates through onomatopoeia, we analyse a corpus of Japanese-style comics (manga), where visuals and verbal text interact to convey onomatopoeia. We argue that the use of onomatopoeia in manga contributes to the recovery of weak implicatures which, in turn, helps to reinforce the persuasiveness of the communicated messages in the examples analysed.