Before heading to Google to find that perfect image (where many images are copyrighted), check out these sites where you can get free images and many are already cited for you, so it's easy to give credit where credit is due.
Pics4Learning - Use the copyright-friendly photos and illustrations for classroom projects, websites, videos, portfolios, or any other projects in an educational setting.
Kids InfoBits - Search for a topic and narrow it down by pictures.
World Book - Use the advanced search to narrow a search to images.
Photos for Class - Search now to download properly attributed, Creative Commons photos for school.
Creative Commons - Search for free content in the public domain and under Creative Commons licenses.