Kathy Schrock has an extensive guide to the best websites. This particular page summarizes the sites she has recommended in the past. (It started more as daily, and is more like weekly now.) Her entire site is very organized, and has something for every discipline.
This links to page 2 of the article, which can be used to show students how to find copyright free images. With students, you can use page 1 to help explain why they need to give attribution, and what Creative Commons is.