Visual images are typically sold by image brokers (commercial vendors) who have made photographic reproductions pursuant to a non-exclusive right with the creator of the object, or who have acquired a reproduction license to market images made from public domain objects owned or controlled by museums or corporations.
Visual images made from reproductions in books and journals for purposes such as teaching or research are understood to be fair use when photographic representations of the objects are no longer available or reasonably accessible from commercial vendors, the object's creator, or the owner of the work.
Assuming a fair use of copyrighted materials in providing images for the purposes of comparisons, contrast, or comment in lectures, permissions are not necessary. Permission is required if the use of the copyrighted image is for other purposes, such as publication, promotion, or in circumstances where profit and/or commercial advantage is the motive for the use.
Continue reading for more detailed information at Policy Section VII. Use of Images
Materials (maps, photographs, diagrams, digital images, reports, books, etc.) created by employees of the federal government while on the job is in the public domain (NOT protected by copyright)—that's why most of the sites listed below are government sites.
Regardless of the site, always check for copyright and use restrictions. Some of the materials DO have some restrictions and as a teacher you need to be aware of them. For example, some of the Smithsonian images must be used as is with no modifications.
Creative Commons – This nonprofit organization’s goal is to distribute for flexible copyright licenses to creators to determine which rights they would like to reserve and which rights they would like to waive for the benefit of recipients and other creators. Search for legal content at this website.
Public Domain Pictures – This is a repository for free, downloadable images.
Flickr – A popular site where user-created content can be put into Creative Commons. Use the advanced search to limit results to Creative Commons content. Millions of pictures are available for your use.
Google Image Advanced Search - Use the Google search engine to find images that can be used commercially, free to modify and free to share. Add search criteria like type of image and format to hone down exactly what you want.