Curt is right. If you post stuff, you can't possibly keep track of who's using it
where, even if you reserve all rights.
here's a pretty good link regarding copyright law:
http://www.bambooweb.com/articles/c/o/copyright.html
basically, your work is copyrighted (protected) from the moment it is created.
of course, it's a lot easier to prove if you actually register the image(s).
a simple: "Copywright 2005, [author's name]. All rights reserved" will serve
to legally protect your rights (remember, you still have to prove you in fact
created the image first). Put this somewhere on your images.
it's redundant, but some people add "Please do not use without author's express
written permission" (that is in fact what "Copyright. All rights reserved by author"
means).
but just because you take the legal steps doesn't mean you're safe. basically,
are you really going to spend a few thousand dollars to keep Joe Schmoe from
using your picture in his website if he decides to be a buttmunch about it?