There really isn't anything you can do if your work is online. There will always be someone, somewhere that will try to cheat the system (and you!). As far as copyrighting your work in general, that will depend somewhat on where you live. The other problem is finding the infringement, finding the culprit and then pursuing it. Is it worth it? As Slobodan suggested, keeping your online images to 1000pixels or less is about the only thing you can do - and possibly watermarking your online images with at least a small copyright notice - 'to keep the honest people honest'!