Unsplash is a great site to use for public domain images. Their terms of service are as follows. Accordingly, Unsplash advertises itself as a location that houses freely usable images. It first began as a Tumblr blog and it has now evolved into a site that houses over 3 million images. Unsplash’s license also states that all photos can be downloaded and used for free. They can be utilized for commercial and non-commercial purposes and no permission is needed but they would appreciate attribution. Additionally, Unsplash has many  images curated for educational purposes.