While you can't create new works of art using just Preview, it is a more powerful editor than it looks at a glance. Just open up any image in Preview, click the toolbox icon, and you'll see a bunch of editing tools. You can easily make backgrounds transparent, annotate images, draw whatever you want as an image, and much more. Preview doesn't have the charm of MS Paint, but it does a bit more than you'd think.
A hidden Mac paint program [Scripting News]