Quick definition: visual rhetoric is a form of communication that uses images to create meaning or construct an argument. Here's an example:
Advertisements are one of the most common and omnipresent forms of visual rhetoric. Have you ever thought about how they work? Consider the ad below, for instance, part of the Got Milk campaign. What does the Hulk have to do with milk?
The answer is: Nothing. The milk celebrities are chosen for their relationship not to the dairy industry but to their audience. If this ad is successful, it makes you want to buy and drink milk not because you trust the Hulk's authority in issues of milk purity or personal health (after all, he is computer-generated), but because you recognize him and his character has a certain cultural currency or popularity that is advertisers hope will transfer onto their product.

That's how it works with all the Got Milk ads -- from those featuring Austin Powers, to those with Hillary Duff, Nelly, Jason Kidd. What is making you buy milk isn't they authority of the celebrity as a spokesperson for milk, but rather their cultural status -- and the fact that they are willing to look ridiculous to sell a dairy product. To a certain degree, the ads themselves have become part of the argument: people are waiting to see who is hip or hot or cool enough to be in the latest Got Milk ad.

Advertisements make these sort of arguments all the time. Think of the ads that use celebrity spokespeople. From Nike (i.e. Michael Jordan) to KFC (Jason Alexander) to Pepsi (Britney Spears), companies trade on celebrity status to sell products. That is, their ads make arguments centered specifically around celebrity appeal.
In analyzing images in this way, what you're doing is analyzing VISUAL RHETORIC: the way the images work on their own and collaborate with written text to create an argument designed to move a specific audience.

Image sources: http://www.whymilk.com/celebrities/hulk.htm
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