I am using Pixen for image editing on the Mac. Like others that have written, I was surprised that no such program was included in the Mac OS. I’ve just barely started using it but it so far it does what I need. I was particularly impressed with the getting started wizard that took you through an overview of the program and allowed you to set some program settings at the same time! I model of what all programs should offer. The only think I would recommend is showing a Page X of Y on each wizard page to let the user know how long it is.
Update: Paintbrush is a nice alternative that emulates the Windows Paint program