The licensing on this website is very simple. Everything you will find here uses the public domain license.
In a nutshell, this means you are free to use them, either commercially or non-commercially, you can also edit them, and you don’t have to credit the source if you don’t want to or are unable to. This is true generous sharing from the photographers at its highest level! This website will always focus on completely free stock images.
To read the license in full please see the creative commons website here: http://creativecommons.org/publicdomain/zero/1.0/
It is worth reading if you are using the photographs commercially to still be aware of some limitations regarding identifiable persons and trademarked products and logos. Certain images may also be subjected to additional copyrights and trademarks. This would require consent of a third party or the suitable license of these rights. A good example of this is when British retailer TopShop used an image of Rihanna on a t-shirt. They had the rights to use the image, but did not obtain her permission to use her face, she successfully sued them. If unsure always be safe and seek legal advice.