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.