If you are a freelance artist, the situation is reversed. Copyright remains yours, unless the company which commissions you gets you to sign over copyright or sign a 'work for hire' agreement.
See this Sample Work for Hire Agreement and this Work for Hire FAQ from the US Copyright Office.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.

