I'd watermark anything I produced. If a huge corp steals a picture that isn't watermarked and uses it then you might not get a penny for it or you'll get sued for trying.
As long as you have the original photo there is nothing that can stop you from suing them for it.
There is no need to put a watermark on your shit to copyright it, anything you've made is yours, no matter what. You don't need to say it, the copyright is there from the start. The only thing you actually would need to say is "Can be used for commercial usage" if you allow people to use it freely.
Of course by saying jack shit there will probably be people using your shit and then trying to say "but you didn't say anything about it on your picture", but that has nothing to say really, though if you don't say anything and they are only using it in a non-profit way you shouldn't be an ass and say they aren't allowed to do it. And if it's something they earn money from friends I still wouldn't complain about it.