A good tip for finding free photos is to use websites such as FreeImages.com. It is important to make sure to check the copyright restrictions on any images you find.
Another good tip is to provide attribution on the free images that you plan on using. Attribution is when you cite the photographer or artist so people know who owns the image. Another good site that has free images to use is Flickr.com. You can find images there by searching and using creative commons license. Dreamstime.com is another useful site for finding free stock photos. Dreamstime also features images that can be purchased for as little as $0.26 as of 2015.