Its a complicated question, and honestly unless you're a lawyer who knows the jargon to deal with copyright, even the law can be confusing about it. If you want to try and go through all the crazy wording, here's a website:

http://www.copyright.gov/laws/

Hopefully this helps! If not, then maybe a comment after you're late morning will make you smile a little,

~Angel~

Not an artist but rather a freelancer so I guess it sort of applies?

The character copyright is the commissioners or whoever owns it. The image copyright is whoever drew it with the exception being if you have signed away the rights as a copyright holder. If you were asked to draw X and literally nothing else, they would own X but not the drawing of X.

In general, and the way I see it displayed on descriptions around here is generally:

"<Character> copyright <Owner>
Art copyright <Artist>"

There are two legally distinct parts of the art going on in there. The owner of the character gives the artist permission to utilize their character for the artist's own art. What results is a shared copyright - the artist can't go around claiming that they own the character, while the character can't go around claiming they drew the piece without either of them being colossal shits.

Unless you have the commissioner agree to give up their "rights" to the commissioned image... you both will share the rights to the finished work. As an artist that takes money for commissions, your performing "work for hire" and the way the law spells out the rights for this can be confusing but essentially under most circumstances it's the "commissioners" idea and your "labor" that makes it happen.

And since your just the "laborer" and your labor was paid for... the commissioner most definitely has legal rights to the finished work. At that state, both you and the commissioner can do almost anything you want with said image (ex: alter, post, etc.) freelely unless you both agreed prior to performing work what the limitations would be.

This is also why professional artwork for businesses are usually done at higher rates as those work for hire contracts require the artist to relinquish all rights to the image upon completion. A good example would be someone commissioning you to create a logo for their thingamajig company.

Characters used within an image are property of their respective owners and by you being commissioned to draw them, your being given a free "license" to use them in the piece. Whenever a piece of art is going to be used for "profit" such as on sellable merchandise... usually additional details are hammered out regarding licensing for either the characters within the art or the art itself.

Bottom line: for 95% of commissions done in the fandom... both the commissioner and artist have rights to the image and are free to use it as they see fit. Many commissioners and artists do NOT have a clue how this stuff works and pissing matches occur frequently. Best way to avoid this is to set clear and consistent terms upfront that commissioners must agree too prior to performing work.