Then be careful who you commission, since your desires do not line up with law and most pro artists know this. The artist retains all rights to the image they create unless they specifically sell it to you and that is in writing. Legally, you have to ask them for permission just to post it, not the other way around.

If you want to secure the copyright to an image and actually own the distribution rights (posting and print-making), be prepared to spend a great deal more than the commission for the right to tell the artist they can not make prints.