Great photos for presentations. Licensed under Creative Commons zero, Unsplash allows you to do anything you want with their photos and does not require attribution.
All photos on StockSnap fall under the Creative Commons CC0 license. That means you can copy, modify, distribute any photo on the site, even for commercial purposes, all without asking permission!
GOOGLE IMAGE search - you can search by "usage rights," and remember that you will need to determine whether or not you need to cite images you find there. For more about that GO HERE.
