The Royalty Free Image License is very similar to the Rights Managed License. The main difference is that with Royalty Free, the client pays for the image only once. After that, they can use the image for any amount of time within the scope of use.
Of course, you can also choose what permissions and constraints you want to apply to your image.
This is probably the best license for you as the creator, because it enables you to exert the highest control over how your image is used.
Most image licensing websites use this model. They apply certain constraints on the usage options. For example, they limit the permitted number of copies, medium of usage (print or web), or the usage period of the image.
Although there may be some restrictions on Royalty Free images, the usage rights are generally way less limited than in Rights Managed Licenses.