It’s your image editor of choice. You’ve upgraded to Leopard. And The Gimp no longer works for you. You can try some of the suggestions found here, or you can just download AquaGimp. Note: I can only confirm AquaGimp works on an Intel based Mac.
With any luck The Gimp will be working in all of it’s X11 glory soon, and this tutorial will become moot.
If you go the AquaGimp route (which is what I’ve resorted to for the time being) continue reading for some tips on a few ‘quirks’ I’ve discovered so far.