FreeDigitalPhotos.net is a new-to-me place to find digital images to re-use for free. FreeDigitalPhotos.net allows you download and re-use low-resolution images without restriction. To download and re-use high-resolution images you need to publish a credit to the creator of the image. That's not a hard requirement to meet. To help you meet the requirement of crediting the photographer, FreeDigitalPhots.net offers a simple chart that outlines how to credit the creator of an image. The left side of the chart lists the ways the images can be used and how to credit the photographer for each use case.
Applications for Education
It is easy to simply right-click on images on the web and save them your computer. Just because it can be done, doesn't mean it should be done or that it is even safe (are you sure that you're only downloading an image and not something else along with it?) and legal to do so. Unfortunately, I frequently meet teachers who allow their students to engage in this practice. Fortunately, there is an easy way to stop that practice. The solution is to use images found on sites like FreeDigitalPhotos.net.
For more free images that your students can use, see this list of sources of Public Domain images.