The reason I ask is because I have to decide whether or not I want one on a book I have coming out next year (my first novel -- trust me, I'll be pestering all of you to buy it when we're closer to the pub date), and I'm on the fence. There are people like my mother-in-law to consider, who says she flips to look at the author photo all the time while reading a book, just to keep steady in her mind the image of the person responsible for the words on the page. It becomes a little totem she can consult and think "hey, good job!" if she's enjoying it or "you suck at writing!" if she's not.
My hesitation is this: the book is a dark YA fantasy, and it seems like very few of these authors include photos in their books. Do we have any fantasy nerds in the house? Why do you think JK Rowling and Suzanne Collins and Philip Pullman tend not to include author photos? My theory is that they feel like real-world photos of some paunchy middle-aged person would pull their young adult readers out of the fantasy they've worked so hard to create. But I'm not certain. Then again, there's Neil Gaiman, who not only includes author photos, but tends to make them as atmospheric and strange as the books he writes (like this one, from The Graveyard Book):
What's the best author photo you've seen? What's the weirdest? What's the worst?