There are many sites where you can use photos and you need to familiarize yourself with the licensing. For example a "creative commons" license means you can use the photo if you properly attribute it. Other sites like morguefile for instance have a collection of completely free to use do what you want with them photos. I have a hub with many of the sites and their licensing policies you can look at if you are interested. Morguefile I believe probably has the most extensive variety - but there are plenty of them. Wikimedia commons is another good place to go and you get photos from there and simply put "wikimedia commons" as the source for example. There are also stock websites where you can purchase various rights to an image, but that can get pricey and there are so many good free resources it really isn't necessary.