YES YOU DO. YES. One more time just to be clear: Yes! The Duck offers free hosting, and a community for webcomics. We DO NOT own any rights to YOUR comics. Federal copyright law is VERY CLEAR on this and is designed to protect the creator. Anybody telling you differently is uninformed or lying. On the other hand, if somebody offers you a contract, such as a purchase agreement or option agreement, be it WOWIO! or Marvel or Simon & Shuster, you should read it carefully, take it to your attorney, and get a clear understanding of what rights you might be giving up once you sign a contract and how much money you may be getting in return. But the choice will always be yours.
However, as someone posting your work on the internet, it is your own responsibility to understand your rights (and the conditions upon which others may or may not use your work).
The Easy Guide to Understanding Copyright: http://www.redbubble.com/people/peterh111/journal/6869928-the-easy-guide-to-understanding-copyright
Frequently Asked Questions About Copyright (US): http://www.copyright.gov/help/faq/Copyright: Basic Facts (Intellectual Property Office, UK) http://www.ipo.gov.uk/c-basicfacts.pdf