It was created by the people behind StoryBoard That, an online storyboard creator, which I definitely need to look at another time. The images are sourced from flickr, with high safety settings. Photos For Class then apply their own filters, and even check the most popular images.
It will search for properly attributed, Creative Commons photos for school. When you download the image (rather than dragging it from browser to folder as I tend to do), it will automatically add to the image the following, and see pink cat photo as an example.
The name of the author
The name of the photo
A link to the original photo
The name and type of license along with a link to read it
Close up below
The full downloaded image is full sized, so will in general need resizing.
Try it out below