These are general places to find images. For art/architecture, photography and science, scroll down.
These collections primarily offer access to works in the public domain. There are no restrictions whatsoever on their use.
These repositories or Web sites offer access to royalty-free, stock or high quality images. Each site has its own terms and conditions under which the photos are licensed. Hover over the link to find out more information on the allowed uses of the photos.
Images licensed under a Creative Commons license are free to use, as long as you abide by the restrictions set forth in the license. At the very least, you must provide attribution.
These Web sites offer access to free clip art, but be sure to check the copyright/license restrictions on each site by hovering over the link.