A complete list of all databases. Sort by subject to see just the databases relevant to a specific subject.
Public Domain, Open Access and/or Creative Commons Image Databases. These are places you can find images that are available for re-use, in other words, you don't violate copyright by using them. But you do still have to cite them!
While not all images on Google Images are Creative Commons licensed, it is possible to limit your search results to only images available under a Creative Commons license. To do so, run your search in the standard Google Image search bar and then on the results page, click on search tools just below the search box. A further drop down menu will then appear with usage rights as one of the options. From there, users can limit results to the proper rights.
This site offers fully searchable access to media, including images, sounds and videos, that has been uploaded by users, mostly for use on Wikipedia. Most of the content is available under some sort of Creative Commons license and licensing information is clearly provided at the bottom of each piece of media's individual page.