FULLY FREE USE
Pixabay: https://pixabay.com/
Pexels: https://www.pexels.com/
Gratisography: https://gratisography.com/ (mostly whimsical and weird)
Jay Mantri: https://jaymantri.com/
kaboompics: https://kaboompics.com/
Life of Pix: https://www.lifeofpix.com/ (counterpart Life of Vids for free video: https://lifeofvids.com/)
Negative Space: https://negativespace.co/
picjumbo: https://picjumbo.com/
Splitshire: https://www.splitshire.com/
StockSnap: https://stocksnap.io/
Unsplash: https://unsplash.com/
Burst: https://burst.shopify.com/
NOT FULLY FREE USE
Vecteezy: https://www.vecteezy.com/
Freepik: https://www.freepik.com/