Royalty–free images aren’t necessarily free. In most cases, you’ll have to pay a one-time fee to obtain the rights to use the image. Then you can use it as many times as you like. The “free” in “royalty–free” only means that you do not have to pay royalties to the owner of the image every time you use it.
Royalty Free is a type of license used by stock photography agencies to sell stock images.
This license type grants the buyer a determined set of rights to use the image in multiple ways, for a flat, one-time fee. This means you pay for the license only once, and you can use it forever within the accepted ways, without any further payment obligations.
A copyright–free image is one that is provided by the photographer to be used by any person. However, the owner of the picture or photographer may still own a copyright on a particular photo or image, but simply has provided authorization for the image to be used by others.