There is no real reason to copy right your graphics unless you stand to lose an obscene amount of money on them. Like for example someone takes a number of your works and makes a calendar out of them. If this is something you are worried about then all you have to do is take your work and send it to yourself registered mail. Then when it comes back sign for it, then leave it alone (DONT OPEN IT). So now you have dated proof by the government that you made your works before someone else did. This will only cost you 2 or 3 dollars. Of course you could just always make the calendar yourself and copy right that.