Some of these pages have only free images and others have a mixture of paid and free images.
Good Free Photos—GoodFreePhotos.com
Can Stock Photo—Canstockphoto.com
Death to the Stock Photo—Deathtothestockphoto.com
Life of Pix—Lifeofpix.com
If you have any favorite sources for free pictures please share in a comment.
This information and more can be found in the resource section of How to Self-Publish: A Guide for Author-Preneurs, 2nd edition by Kayla Fioravanti.