There are many images available on the Internet that are free to use for school work. Please note that these are for school use only. See the website use policy for any other uses. 

US Fish and Wildlife Service - images of birds and animals

NASA image gallery - images of planets, solar system and space travel

N.O.A.H. photo library - images of weather, oceans, people, space, and more

Classroom Clipart - free images in a variety of topics for education use.