Copyright does not have to be purchased - one's work is immediately copyrighted as soon as it is created.

To "copyright" my pictures, I add text in photoshop. I'm not worried about anyone reposting my photos or what not - but rather I'd just like to have my name on it if someone decides to use it. (Such as my friends like to re-post my photos in their livejournals or myspace from a night we hung out, etc.) If they decide to edit it out, oh well.

I HATE watermarking photos. I think it takes too much from the photo, and I'm not that worried about it.