Just my 2c...
Ah yes... Legal matters and the photographer... As Bram says, unless you sign the full rights away, all your creations are yours... until someone steals them that is - voice of experience speaking here.
There are a number of tutorials online about copyright by law firms and I recall several are referenced somewhere on here too.
Are you prepared to take a copyright case to a Federal judge, because some nut case in the middle of nowhere put a copy up on their personal web site? Remember, the costs of bringing a federal case are as much as most people earn in a year and no other (or lower) court will consider one.
Be as angry over picture theft as you like, but if you can't actually get a legal remedy, consider the value of being angry...
On the subject of action:
Should you register them (and get properly worded release forms signed by your subjects)?
Picture registration is something a lawyer will tell you it is essential and to spend loads of money on doing - through them (for a fat fee) of course... ;-) It depends on how 'formal' your registration needs to be - to stand up in a court case needs more than just putting a (c) symbol on it - even if that alone *might* help... as they say, "Your Mileage May Vary" and what you *might* get back will increase the more formal you get!
You can easily (and relatively cheaply) produce a cease and desist letter, asking that they either credit you as the photographer or take it down in a nice friendly wording that doesn't piss them off and gets you a plug online (voice of experience here again).
It all helps if you can get them to link to your web site as a result... Remember those Google stats benefit from links to your site etc... Work can come from this kind of thing...
If the offender is someone like Fox or the New York Times then there are a number of well established cases of photographers winning - as then it can be well worth it - and also of photographers losing...
You might like to read the reports in the British Journal of Photography (BJP in the UK) and it's well covered feature of the case with the photographers gallery as a typical (yes TYPICAL) example to just how dumb all this can become.
As someone who has had wasted a week of his life embroiled in legal crud about this kind of subject (a pic of mine was copied and used in some brochures some time ago without my permission), I suggest "byancave" LLC (see dot com for the link), who do a great and easy to read and use FREE 12 page primer in PDF format on this subject... We are not supposed to post links to external web sites, but you can guess what it is from this...