Pdphoto.org: Public Domain Photo supplies a range of images, though I'd argue the search function is a bit inaccurate and the quality of images is not high.
Niche Photography uk: Images are location-specific and free and easy to download; the photographer asks for a link as credit.
Chelsea Blacker, head of client delivery at digital marketing agency Blue Glass UK, put together the following list of websites that provide free images for commercial business use, which means an image is available through a Creative Commons license or is royalty-free.
Turning OFF the Family Filter may display content that is only suitable for viewers OVER 18 YEARS OF AGE.
Morgue File: Some great images, but you must click on each image individually to determine if it's available under Creative Commons.
I found about one in seven images was CC, but many of those were still not available for commercial use.
USA.gov: By far the most random site on this list, the U. government has provided a list of sites with images on offer from such sources as the National Archives and NASA.
warns that not all images on the sites are in the public domain, so double check.
Flickr Search Engines John Johnston.info: Notably simple, pulling from Flickr.