PublicDomainPictures.net is a repository for free public domain images. Material can easily be browsed by various categories, including professional or seasonal photos. Image tags allow an easy search.
2- Wikimedia Commons
This is a database of 18,083,944 freely usable media files to which anyone can contribute. Media files can be surfed by topic, location, type, author, license, and source. Each of these categories has subcategories, to refine the search.
Pixabay is another great resource for high quality public domain pictures. You can register with pixaby and save yourself the trouble of having to type in captcha each you want to download a pic.
4- Public Domain photos
All photos on this web site are public domain. You may use these images for any purpose, including commercial.
The morguefile contains photographs freely contributed by many artists to be used in creative projects by visitors to the site.
This is another great resource of free images and similar to Google Images, Flickr offers a search functionality that you can use to look for only public domain pictures. Refine your query in Flickr using advanced search to look for and find free images.
7- Image After
I am also recommending this great list from edtechteacher featuring a variety of other websites where you can find public domain pictures.