Here are some great image collections that give you permission to use their pictures.
Creative Commons Search - search for images that you have permission to use in projects.
Clippix Etc. - Great photographs from Educational Clearing House. Organized by category.
Photos for Class - searchable database of age appropriate photos for class projects.
Pics 4 Learning - Large collection of photographs organized by category.
Pixabay - searchable database of high quality images
Classroom Clipart - A collection of clipart and photographs. Organized by category. Has a section just for the United States (includes a link for each state).
Clip Art Etc. - Black and White drawings. Organized by category.
NOAA Photo Gallery - National Oceanic and Atmospheric Administration