Some of the images cost a small fee to download and use. But there are tons of sources to find free legal-to-use photos as well. For example sites like Flickr, stock.xchng and depositphotos are some of the popular choices. While some sites may require you to sign-up (for free) to gain access to both their collections of paid and free images, it's important that you still comply with the user's common creative licenses. This will specifically state how the owner wants you to attribute the photo and under what terms it can be used.
You may also use public domain photos. Typically images that fall under this category include the following: photographs and pieces of art work created before 1923, photos created/taken by the U.S. government, photos that have an expired copyright, and photos that are used on educational sites like Wikipedia.