I might be the last one to get to know about this site, but still mentioning it here for others to find.
https://unsplash.com/ is a super active community for photographers, where they share amazing photos for the world to use for free!
As stated on their licensing page:
All photos published on Unsplash can be used for free. You can use them for commercial and noncommercial purposes.
It's of course always appreciated to attribute the photographer as well as Unsplash, but it is in no way required. This will definitely be my go-to source for presentation backgrounds etc.
Side note: There's also https://undraw.co/ with some nice vector drawings! :-)