What is a visual metaphor?
A visual metaphor, also called a pictorial metaphor, is a metaphor in which something (the metaphor's "target") that is presented visually is compared to something that belongs to another category (the metaphor's "source") of things than the first, also presented visually. As in verbal metaphors (such as "football is war" or "the world is a stage"), at least one feature or association is "mapped" from the source to the target. Often, a whole set of (interrelated) features is mapped from source to target. Visual/pictorial metaphors are used often in advertising, but also in political cartoons and films. Many examples of visual/pictorial metaphor, as well as discussions of them, are discussed in my book Pictorial Metaphor in Advertising (Routledge 1996), which also contains references to the work of other authors who discuss metaphor in images and film, for instance the perception psychologist John Kennedy, the film scholar Trevor Whittock, and the film philosopher Noel Carroll.
Nowadays, metaphors straddling two or more modalities (language, visuals, sound, gesture ...) are beginning to receive serious scholarly attention. Metaphors in which the target and the source are in different modalities are called "multimodal metaphors." An example of the latter is an advertisement for a photo camera (target, in the visual modality) with underneath the text "supermodel" (source, in the verbal modality). For more information, see my online course *A Course in Pictorial and Multimodal Metaphor.*
In September 2009 the volume Multimodal Metaphor (Mouton de Gruyter) appeared, which I co-edited with Eduardo Urios-Aparisi. More information on this topic can be found on the Adventures in Multimodality (AIM) blog [Contribution by Charles Forceville.]
A mistake in a contact lens prescription is a visual metaphor for Oedipus in "Oedipus Rex" by Sophocles (495 B.C.E. - 405 B.C.E.). Specifically, Oedipus is mistaken about his self-identity. That his start point is off makes everything else off in his life. The same may be said of an individual whose contact lens prescription is incorrect.
Best examples are those in which the picture is a blend of elements of two unlike objects. Real examples from advertisements: a bottle of Absolut vodka covered with a cucumber beauty mask (with the slogan 'Absolut beauty'; both the slogan and the picture present the vodka as a -female- person); a car ad that showed an elegant shoe with wheels (suggesting I suppose that the car is elegant, comfortable and cheap like a shoe) etc…
In the metaphor 'The sentinel tree stands guard at the gate' why does sentinel get compared to a tree?
Visual aspects, such as speech/thought bubbles, the panel, the gutter, captions, and sound effects, are important in understanding as well as other elements. Transitions, imagery, diction, characterization, emotion, style, symbolism, metaphor/simile, setting, plot, tone, and theme are all present in a graphic novel. However they are communicated through dialogue and visual elements (the actual illustrations).
An extended metaphor is a metaphor that is started early in the work, usually at the beginning, but it can be added later, that evolves and stretches itself throughout the passage. For example if you were to write a paper, and use a road as a metaphor for the path of life, and you were to develop and use this metaphor throughout the paper then it would be an extended metaphor.