What is Royalty?
Consideration paid for a license to intellectual property generally based on some measure or percentage of revenue or profit realized from the intellectual property by the licensee.
A type of contract between two entities (the licensor and licensee), that is employed when licensing the rights to use content, such as photographs.
The term Royalty Free means that once the content is licensed under a set of guidelines, the licensee is normally free to use it in perpetuity without paying additional royalty charges.
Is "Royalty Free" free?
Though these sites will provide "royalty-free" images, the images themselves are usually not free. Royalty-free simply means that you may use the image without paying for that use. Because the types of usage may be limited, read the fine print at each site.
Can I use someone elses work?
Even if things are not Royalty Free, you may still be able to use them. Here are the suggestions about copyright use from from the U.S Copyright Office.