This is an interesting article on the subject. It has a pretty funny discussion of how the author is costumed and posed to represent the genre, e.g., weird (Stephen King), bookish (Ian McEwan), feminine (Danielle Steele). Dean Koonz always has his dog Trixie in his author photos.
Why authors photos appear on books
Here's an excerpt. I think I agree with most of this.
"Jim Cox of the Midwest Book Review recommends that publishers include author photos as a marketing tool for three main reasons:
Author photos help to sell books.
Author photos help to build name recognition.
Author photos help TV bookers decide whether the author would be "good television" material.
This is very bad news. But Cox delivers it like a messenger who doesn’t understand the telegram. He continues:
We, as a species, are visually oriented. Nature made us that way. We get most of our sensory data through what we look at. We are conditioned to notice what our fellow homo sapiens look like. We are usually unconscious about how great a role the visual impression of others has upon us in making response judgements.
The "response judgements" to which Cox refers are purchasing decisions. Book buyers are generally unaware of how their visual impressions of a writer affect their choice to buy his or her book. Such is the nature of subconscious manipulation. For that matter, such is the nature of marketing. Playing to our visual sensors is not a new ploy invented to part readers from their money. As a society, we rely on visual shorthand for information in every sphere, from educational puppet shows to television news. In this case, however, the consequences of leaning, even subconsciously, on an unreliable visual crutch (the author photo) to assess the value of an intangible product (the unread text) compromise both the consumer and the producer and in effect, change the industry. And not for the better."