Given the nature of fixed visual images, visual rhetoric is usually seen as monologic. However, in some cases, an analysis of images can reveal them to be dialogic in a weak sense. This hypothesis is examined taking as examples political images, mostly posters, protesting against war. One obvious possibility consists in introducing a written dialogue in the poster. More interestingly, in many posters there is a play between text and image: since an image cannot directly negate, it often shows the crude reality of war. In these cases, it is the text that negates what is affirmed by the image. The opposition between images showing the destructiveness of war and texts rejecting war is analysed both rhetorically and in terms of Ducrot’s theory of polyphony. Finally, parodies and pastiches provide good examples of a kind of dialogue between an image and its source.