Creative Common's image source.
If you need an image that is licence free for commercial/non-commercial use, you can search for one in the Creative Commons database. https://search.creativecommons.org/
This way, you're searching for images that have granted a legal permission to either allow modifications/adaptions/rendering and/or for commercial use without having to pay for royalties. Also, most of these are available for free to download. Some might pay a flat fee to the artist as a compensation.