There are several places online to find freely usable photographs. Flickr, Google Images, Pixabay and Wikimedia are some sites that offer photos in the public domain or licensed for public use through Creative Commons.
Flickr, Google Images and Wikimedia offer a large range of photos under a Creative Commons license, which allows for various different uses with the photo's creator's permission. Requirements for using Creative Commons photos usually include attributing the photo's creator and linking to the original photo.
These resources also offer assorted photos in the public domain, meaning they're free to use in any way, without crediting the photo's creator. Pixabay is another site that offers photos in the public domain.