There are many places online to find free images that you can use. To be safe, remember to always double-check the licensing conditions of individual items. Here are some sites that we recommend:
Creative Commons Search
A meta-search tool. Can be used to find CC-licensed images on Google Images, Fotopedia, Europeana, etc.
Flickr Creative Commons Search
20 million freely usable image, sound, and video files. All files are Public Domain or CC-licensed. Check the “Licensing” section for an image to find any terms and conditions for reuse / instructions on attribution.
The WorldImages database provides access to the California State University IMAGE Project.It contains approximately 100,000 images, is global in coverage and includes all areas of visual imagery.