Pixabay

RIght now I find that the free library at https://pixabay.com/ is great. Their Blog Post describes in detail what can be done with the pictures:

• All images on Pixabay are released into the Public Domain under Creative Commons CC0. Therefore, the images can be modified and may be used freely for any application - also commercially and in printed format. Attribution in appreciated, but not required.

Exceptions:

• Identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent.
• Do not suggest endorsement of products, services, etc. by depicted people or organizations. For example: do not use an image of NASA and place it adjacent to your own product in a way that suggests NASA would recommend the product.
• Conclusion: It all may seem terribly complicated or risky, but actually, it isn't. Simply put yourself in the position of a depicted person or in the position of an owner or designer: Would you approve of the intended application without being asked? That is the question you should always ask yourself before using a public domain image without release.

Understand "public domain" as the permission to freely use (display, modify, print, etc.) an image without asking permission from the image author. However, it is still your responsibility, to make sure the depicted content (persons, logos, private property, etc.) is suitable for your application and does not infringe any rights.