As the copyright holder, you are provided exclusive rights in your work as per the Copyright Act. Here in the US, as in many countries, copyright is automatically granted to you as soon as you create a piece of art, but registering the copyright means that your ownership is a matter of public record, and makes it easier to defend your rights. For instance, if you want to bring a lawsuit for copyright infringement, you will need to register your copyright. Those rights include the right to make copies, distribute, publicly display, and make derivatives of your work. Those rights are solely yours and cannot be taken away. However, those rights can be transferred to another entity; whether in total or just a few of the rights, for a limited time or for life. With a license, you are not “selling a work” as such. What you are doing is selling permission to use that work in a certain way for a certain length of time.
For information or questions on art licensing opportunities