When you are working on a website, ebook or other type of digital or physical product needing graphics, you can use domain public images, but as they are public is possible everyone is using them too. In this case, it’s better to use royalty free images, that’s, images that you pay once and can use how many times you want.
If you search for royalty free images, you’ll find several stock photo sites, like GraphicStock, ShutterStock, Fotolia and GettyImages. Some of them contain only paid content, so you need purchase credits and use credits to get images. Others containt free content, but you need include a reference to author with one URL in webpage containing that image.
When looking for images to use in your projects, don’t limit yourself searching only for “royalty free images”. Search for “stock images” or even “free stock images” too and you’ll have more chances to get an image you wish.