Photos are copyrighted the moment you take them but it's best to register them with the
gov and it affords not better protection but better payout of you catch someone doing this. There really is no way to totally protect yourself from image theft if you have them posted on the web. But using small res images, marked with copy helps. The PPA has also gotten many of the places that would normal copy a photo for you to no longer do that without the photographers permission. Another site I like is this one
PhotoAttorney she has a great deal of helpful information concerning all areas.