FreeToUseImageLinks
https://www.flickr.com/search/advanced/
http://commons.wikimedia.org/wiki/Main_Page
http://creative-commons.deviantart.com/ looks like a great place to start - it also gives you a workaround of how to search the whole of deviantart for CC works.
If you go to google images and to the Search Tools button there is a dropdown for usage rights. Click "Labeled for reuse" and it searches across a bunch of these sites we've mentioned and also some others e.g. public-domain-image.com, pixabay.com and geograph.org.uk
The motherload: http://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources