On this page there are sites for finding Public Domain and Creative Commons photos. Although Public Domain Images require no attribution, it is best practice to include a link to the photo and to attribute the photographer and the website where you found the photo (e.g., Photo by Jeremy Bishop on Unsplash) . Creative Commons Images have seven different licenses. You must pay attention to the license acronyms and cite accordingly. Below is a chart, found on Wikipedia, that describes what can and cannot be done with images that have the various licenses.
These pictures require no attribution but it is best practice to give credit by including a link to the photo and credit the photographer and the site where the photo was found. (E.g., Photo by Jeremy Bishop on Unsplash)