Locating and using openly licensed, creative commons, or public domain content is a great alternative to requesting permissions or doing a fair use analysis on copyrighted images or other educational content. The following sites are a good starting point for your search:
A site that provides free, public domain images that don't even require attribution. Unfortunately, it also offers Shutterstock images, which aren't free, but they are clearly set off so I don't think users will find that too confusing. The search feature is particularly nice, supporting Boolean connectors and also the ability to search by predominant color of the image.
A collection of two million plus digital images in the fields of the arts, architecture, humanities, and sciences. Designed for high quality use in presentations and instruction. Software allows for zoom and download. Some quicktime panoramas.
Presents thousands of digitized images of varying sizes from the Prints & Photographs Diviison of the Library of Congress. Diverse content includes baseball cards, Japanese-American internment (Ansel Adams), African-Americans, architecture, posters, and much more.
Oregon Digital is an extensive repository for digital items created by the libraries at the University of Oregon and Oregon State University. Included are almost 100 individual collections covering a variety of subjects scanned from diverse original formats.