This list is a further update to the one I posted back in April 2016. There has been a few more sites I have found since then:
- PEXELS -- https://www.pexels.com/
License: https://www.pexels.com/photo-license/ - Unsplash -- https://unsplash.com/
License: https://unsplash.com/license - RGB Stock -- http://www.rgbstock.com/
License: http://www.rgbstock.com/license - New Old Stock -- http://nos.twnsnd.co/
License: http://nos.twnsnd.co/rights-and-usage - Jay Mantri -- http://jaymantri.com/
License: http://creativecommons.org/publicdomain/zero/1.0/ - Public Domain Archive -- http://publicdomainarchive.com/
License: http://publicdomainarchive.com/terms-conditions/ - ISO Republic -- http://isorepublic.com/
License: http://isorepublic.com/terms - Gratisography -- http://www.gratisography.com/
License: http://www.gratisography.com/terms.html - Picography -- http://picography.co/
License: http://picography.co/terms/ - Pond5 -- http://www.pond5.com/free
License: http://www.pond5.com/legal/publicdomain - Pixabay -- https://pixabay.com/
License: https://pixabay.com/en/service/terms/#usage - StockSnap.io -- https://stocksnap.io/
License: https://stocksnap.io/license - SKUAWK Public Domain Photos -- http://skuawk.com/
License: http://skuawk.com/license.html - BURST -- https://burst.shopify.com/
License: https://burst.shopify.com/legal/terms - Mockup World -- https://www.mockupworld.co/all-mockups/
License (Check with individual creators): https://www.mockupworld.co/faq/ - Original Mockups (Free category) -- https://originalmockups.com/free-mockups
License: https://originalmockups.com/support/file-licenses - FocusFitness -- https://www.focusfitness.net/stock-photos/
License: Creative Commons Public Domain
Note: Remember to always check the licenses offered by the stock image sites, as their terms may have changed.