I have had several chairlift discussions about the implications of making images available on the internet and whether those images can then be used by third parties (possibly maliciously etc).  It turns out that there is a fair amount of information about this on the internet - not surprisingly...

The best article I have found describes the allowed usage of information in USENET use groups, but if relevant to all information found on the internet:

http://www.templetons.com/brad/copymyths.html

Read the summary at the bottom of the article for the details, but to paraphrase, all information posted to the internet is copyrighted to the owner and cannot be reproduced by any means without permission of the owner, whether the information has been tagged with a copyright notice or not.