Foto.com is an extensive, easy to use CC image repository which provides ‘ready to paste’ attribution information for use with the photographs.
Pixabay Image provides free public domain images. Check the usage terms of CC license before downloading.
Wikimedia Commons is a free media repository. Please check the type of CC license and its usage terms before downloading.
Wikipedia : Public domain images lists a large number of sources for images which lists a number of public domain images on the web. Users are responsible for checking the copyright status of images before using them.
Morgue File has free reference images. Please note that when a search is displayed, the tabs offering alternatives to the free photos on Morgue File are not free.