Under current U.S. law, you automatically own the copyright in each work you create. This does not transfer to a purchaser. The purchaser only retains the right to display the work sold to them in the original medium. The books I reference frequently are the books by Tad Crawford, such as
http://www.amazon.com/Business-Legal-Forms-Fine-Artists...d=1211862841&sr=8-2, and the Graphic Artist Guild's Handbook
http://www.amazon.com/Graphic-Artists-Guild-Handbook-Gu...id=1211864055&sr=8-1. I am not a lawyer, what I state here, is my interpretation based on my research. If you have doubts you should contact a lawyer.
You should have a contract every time you sell a work, even if it's a simple purchase agreement detailing the copyright and reproduction right terms (i.e., artist retains copyright in the work, buyer must seek prior permission from artist for any reproduction of work, buyer must take care of the artwork, etc.). If you are selling colored pencil works for wine company labels, you should have a contract saying that you retain copyright in the work, and you are authorizing the wine company to use the image on a non-exclusive basis, and state the territory, imprint #, and length of license, etc. The broader the territory (i.e., do they sell in California only, the US, worldwide...), the greater the imprint amount, longer the license, the more you would charge for usage. You would NOT just be charging them to buy the original 5"x7" image. You are charging them a licensing fee, which is quite different.
Also, if you haven't heard about the Orphan Works Bills of 2008, currently before the House and Senate, please take a moment to review some background info on
http://www.illustratorspartnership.org/01_topics/article.php?searchterm=00185. Under this bill you would be required to register each work to keep your copyright. Currently copyright is automatic. This bill affects every artist and would cause hardship to every artist, due to the time and expense involved in registering each work. And this bill would apply to all works created in the past 30-odd years, not just future works.