Here are a few resources:

Pixabay: https://pixabay.com/
Flickr: https://www.flickr.com/
Pexels: https://www.pexels.com/
Unsplash: https://unsplash.com/
Freepik: https://www.freepik.com/

Capto: https://www.globaldelight.com/capto/ (This is the one I use, but there are other options)

Explore more: https://www.presentationzen.com/