Royalty free images have a fixed price based upon the size of the image.
(This contrasts to "Rights protected images" that are rented for a specific, one-time use at a price that is determined by the scope of this particular usage).

When you purchase a royalty free image (or a cd-rom containing royalty free images), at a fixed price, you can then use it as many times as you want without further payment.
See : Usage rights of Royalty fre images
But you have no ownership rights whatsoever to this image. Content providers (photographers, models or publishers) remain the sole owners of the copyright.
The terms and conditions of the license explains under what conditions this autorisation use is granted and serves as your contractual guarantee.

Please carefully read the User License Agreement.

See : Pricing and Technical Information