Ever heard of Pixelmator? It's a recently released image editor for Mac, with a cool Leopard-friendly interface and lots of tools and effects to work with. I'd probably consider it just a notch under Adobe Photoshop, because it doesn't have as many effects, plugins or tools to work with, but it's probably a little more user friendly and still achieves great results. What I really like about it is that it seems to have been designed specifically for Mac, so it works great with features like Automator or iSight and features that nice dark grey Cocoa interface. Pixelmator can be used to edit pretty much any sort of image or photo. If you're interested, the developer even has a blog, where he regularly writes about updates and new features for Pixelmator.
I recently found a cool tip on how to make an Aurora Borealis with Pixelmator, which looks great as wallpaper. What's an Aurora Borealis you might ask? It's a luminous display with strange shapes and vivid colors, usually reds, purples and greens, that you sometimes see in night skies. In the Northern Hemisphere, aurora borealis are called northern lights and in the Southern Hemisphere....southern lights. They mostly occur during equinoxes. Aurora Borealis are a real phenomenon to see and very much studied by researchers and scientists.
Check out the easy step-by-step guide from the Abduzeedo design blog to making an Aurora Borealis in Pixelmator. You'll learn how to use tools like the lasso or gradient and filters like gaussian blur or zoom blur in just under 10 steps and 20 minutes. Some will say that it pretty much resembles the default wallpaper on Leopard. True, but at least here you can tweak the color, intensity and shape of the Aurora Borealis. Also, it's a great way to learn how to use the Pixelmator image editor.