My fiancee is an artist, and regularly creates digital art work, mainly digital drawings and the occasional print of that artwork. There is a certain level of implicit 'copyright' of the artwork, as I understand it, as her intellectual property.
However, individuals occasionally commission her, which goes a little outside my understanding of these implicit IP rights in the circumstances. I also occasionally commission artists to create artwork, of which the subject matter, situation depicted, and in most cases the characters depicted are my intellectual property (or, at times, the joint intellectual property of me and my fiancee).
This is essentially the question at hand: Under United States law, where we reside, what are the legal rights and protections granted to an artist for their artwork which they create, and are those rights and legal protections lessened when they are commissioned by another individual, depicting characters, individuals, or situations which are the intellectual property of the one who made the commission?