Ookaboo is easy to use and advertises itself as a place for free pictures of everything on earth!
FreePik is just as comprehensive and allows you to search for vector as well as photo images. My students have enjoyed this site.
Creative Commons is a comprehensive site that searches multiple images.
Imgur is new and requires registration but it looks awfully easy to use.
Wylio is my complete favorite! It searches for the copyright free image and allows you to edit it right at the site. It will also generate code to be embedded on your website or blog.