A subsearch of National Geographic. If you are looking for primary source photographs, these are beautiful. They have a direct link for downloading photos to Diigo. Many categories of photos. This particular site on rainforests would be useful to elementary school students doing research on environments, animal habitats, etc.
This site is great for K-12 students. It contains current event articles on a large scale of topics to keep high school students engaged in the world around them. There is a special "Kids" page with interactive games and lessons for teachers. This is a phenomenal site for educators.
Hundreds of free activities and lesson plans for educators in the areas of social studies and geography. Also includes printable maps of nearly every continent and country. The interactive museum allows students to take geography journeys to mountain ranges, archaeological digs, outer space, and other animations.