Finding Images
There are libraries of free images available online
WikiMedia
Content under open content licenses may be reused without any need to contact the licensor(s), but just keep in mind that:
- some licenses require that the original creator be attributed;
- some licenses require that the specific license be identified when reusing (including, in some cases, stating or linking to the terms of the license);
- some licenses require that if you modify the work, your modifications must also be similarly freely licensed
Creative Commons
You can use CC-licensed materials as long as you follow the license conditions. One condition of all CC licenses is attribution. How to give attribution: https://creativecommons.org/use-remix/get-permission/