I know I'm late with this topic, but I find it quite interesting.
If you check the link JerseyCaptain has given, you'll see that it is a long and complex issue. And
extremely confusing. If you don't spend time reading up on it, you won't understand it. Trademarks and patents are other things entirely applied to the same issue.
1) If you post something of your own creation on the internet - it is yours by law. However, this applies to the completed work. Characters, however, cannot be copyrighted. In that case, you'll have to look into trademark law. Mickey Mouse, for example, is a Disney trademark. (I don't know a lot about trademarks or patents so I'm not going to claim any knowledge on them, but by all rights I think they have the same protections as copyrights - Disney has gone after trademark violators as often as they have copyright violators).
2) If you post something that contains material that is copyrighted to others, you cannot hold the full copyright. For example, if you use a photograph or a still from a film, you may be able to make an icon - but you cannot claim full credit.
3) Artwork, photographs, stories, etc that are completely out of your own design are legally yours - once they are 'published' - posted that is, on the Internet. This is under US Copyright law, though it is recommended that you register the work. This also means that if you post something whether it is a photograph, artwork, story, etc that belongs to someone else you can get into serious trouble.
(And here's two more links that may help if you have further questions:
http://www.copyright.gov/help/faq/faq-general.html and
http://www.whoishostingthis.com/resourc ... ght-guide/ )