I, the artist, hold every right to the produced drawing, not the commissioner.

Therefore I´m allowed to use the copyrighted artwork to;
Promote myself with in any place or site.
Publish books with.
To display it anywhere to my liking.
Post it wherever I want.

The commissioner are allowed to;
Use the copyrighted artwork for personal use only unless agreed otherwise.
Print the art, and claim the right of their character(s) but not the drawing itself.
Use the art to promote themselves with proper credit given to the artist.

If you break the copyright infringement in any way, every right you had to the drawing will be retracted.