Stock photos, from such sources as Fotolia, ShutterStock and iStockPhoto are an excellent alternative to expensive custom photography for book covers and interior pages. Millions of photos and illustrations are available from skilled pros and amateurs for a tiny fraction of the cost of hiring a photographer and models and renting a shooting location and paying for travel and food.
Unlike some "stock photo house" policies aimed at periodicals rather than books, you are buying a license for nearly unlimited use. You don't pay more money based on the readership/viewership of your media, or the purpose of your project. All the files available I've used are royalty-free, meaning they can be used with no limits on time, number of copies, or geographical location.
Most of them were absolutely perfect, and could not have been more perfect if they were shot just for my books.
Since they're not mine exclusively, I check to see if any competing or related books have the same or similar illustrations. There is no guarantee that one won't go on sale in the future (there's also no guarantee that another book won't have the same title as mine), but I'm willing to take the risk.
About the only limitations are that you can't put any person in a photo in a bad light or in porn or a violent situation, or use a photo to support a political party or religious organization.
One other possible limitation is that despite a nearly endless selection from Fotolia and its competitors, you may not find a photo that's exactly right for you. Read the contract, but you are probably allowed to modify a stock photo to make it 'more custom' (and more perfect) by flipping, cropping, changing lighting, removing or adding background, etc.
That's where someone skilled with Adobe Photoshop can remake a stock photo into a custom photo.
The photo in the book cover at the top was nearly perfect, except for a generation gap. I needed a picture of a father speaking to a child, but the original man (in inset on the right) was obviously old enough to be a grandfather, or even a great grandfather.
Carina, my ace cover artist, gave him a hair transplant, eliminating the effect of decades.
In the second row, Carina doctored my 1971 wedding picture, to remove my wife and remove a reddish cast from the photo.
In the bottom photo, Carina removed a cluttered background, straightened out my tilted head, and removed my right hand that looked like the deformed appendage of a Thalidomide baby.
WARNING: some collections of stock photos and "clip art" are not supposed to be used for commercial purposes -- like books -- so read the fine print carefully.
OTHER WARNING: Stock photos are often purchased with "credits" that you buy in batches using a credit card or Paypal. Credits don't last forever, and can expire before you have a chance to use them. Be careful. BigStockPhoto has a "pay as you go" plan that allows you to buy what you need when you need it and not risk having credits expire.