Large parts of commercial art, including the history of comic books, are and were done as work for hire, with no ownership of the art retained by the artist. Just as a carpenter does not retain an ownership stake or attribution in the creation of the architect who employs him. Recently comic book artists have retroactively asserted their rights as creators over and against their initial contracts to produce work for hire, and they have won some actions in court. It’s always an uphill battle to renegotiate an employment contract, and money awarded have often been minimal. Movie makers employees such as soundtrack artists, cowriters of scripts, are often given a choice at the signing time between a percentage ownership and/or a onetime contract fee.
Your best option is rarely obvious, and being offered work for hire is not necessarily a bad thing. Here’s when a hard boiled and experienced (and trusted) manager/representative can help you.