Royalty–free (RF) material subject to copyright or other intellectual property rights may be used without the need to pay royalties or license fees for each use, per each copy or volume sold or some time period of use or sales.
Many of these photographs are free from copyright restrictions or licensed under creative commons public domain dedication. This means you can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.
However, some photos may require attribution. You are advised to do your own research and determine how these images can be used.