Open images are available for use on the open web and other unrestricted environments. You are free to use them in multimedia projects, websites, blogs, portfolios, etc, that are open and available to the public. When you use open images, you do not need to restrict access to your class or academic environment.
Always read and comply with the use restrictions for specific image sources. Always cite images someone else created.
Images should not be sold or used in commercial products or for commercial purposes.
Flickr contains millions of photographs shared by Flickr users under Creative Commons licenses, and is a strong source for contemporary travel, nature, people, and design photographs.
Creative Commons is a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing." (http://creativecommons.org/about/what-is-cc).
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.
These sites offer at least some images for free general use. These sites also contain advertising and/or offer images for sale.
Read each site's conditions for use carefully. Different images from the same site may have different use restrictions, too. Make sure you know what they are.