Just a few words regarding what can be a complicated subject. Most artists in the majority of cases hold onto copyright. It is only fair that there should be protection against indiscriminate exploitation of a particular work.
It is true that the majority of clients and individuals are mindful and understanding of this.
If I were to provide a portrait to a customer then they own the work of art, but not the freedom to exploit or sell printed versions from that original. At a more commercial level I would always discuss the potential usage of a specialist scene or illustration and then tailor a copyright license to include these terms. This might include a one off usage in a leaflet with a small print run, to a promotional poster marketed and displayed throughout the United kingdom. The price will reflect the usage so that it is fairer to illustrator and client alike.