While these websites do not actually contribute image content themselves, they’re able to index images in a way that makes it easier to search for free content. Because the images come from a variety of sources, it is important to note that these websites tend to have a worse track record when it comes to filtering inappropriate content for students.
When it comes to searching for images for my blog, Bing is the clear winner for one simple reason; while Google and Yahoo also allow you to filter by license types, Bing is the only one that will search strictly public domain images. You can’t beat public domain when it comes to blog images. You can freely modify images, use them commercially, and no attribution (or research into attribution) is needed....
Older resource for visuals but Dan Leeman's sources for free images are organized to help you find what you’re looking for. And he has a list of terrific niche sources like photos of the earth, archives and categories of all types. My current fav is Pixabay!