Gotta agree with lbj here... we don't know why you need one, so coming up with reasons to convince your parents is impossible. You know them better than we do. You know why you need (more correctly, want) a MacBook better than we do. You've bartered with your parents before -- we have not.
Simply saying, "I'm taking courses in graphics" isn't enough for us to fabricate lies to tell your parents to trick them into purchasing a computer that you do not need for your studies. You've already got several Macintosh computers -- what's wrong with them?
The short answer is that we can't make up anything because you're the only one that knows the real reasons. Tell us the real reasons, and we can help you formulate those reasons into a valid argument that you can then take to your parents. Until then, though, you're pretty much on your own -- I don't think there's anyone here with morals low enough to help you come up with half-truths and lies to finagle your parents into spending $1000 they don't need to spend.
And besides -- there are a lot of ways you can make money at 13. Paper routes. Lawn mowing. Chores for the neighbors. Lemonade stand (thanks, nixgeek!). Sell off old toys/computers/clothes/stuff. Garage sale. Those are just off the top of my head -- things I did as a kid to make a buck or two. I'm sure there's infinitely many more things one could think of if they spent 5 minutes trying.
If you're having this much trouble finding ways to make money, then I highly doubt that you'll be convincing anyone anytime soon to flat-out buy you expensive stuff. I personally think the, "Hey parents, pay for half, I'll work off the other half and bring home excellent grades" idea is decent. If you can convince them it'll help you get better grades (which I'm doubtful of), there's not many better reasons a parent would like to hear.