For vector-based images, use
Xara.
The site's suffering from a slashdotting, so it might take some time to load. But Xara is an opensource, cross-platform Vector-Based art creation program that rivals Adobe Illustrator.
The actual implementation of vector objects is a little more complex than the standard images that we all use, but actually making vector art is not really that much different. In fact, it tends to offer more flexibility in terms of what you can do with it. Its anaologous to procedural content generation.
The buttons and commands for Xara or Adobe Illustraotr are basically very similar to Photoshop or Gimp. Not a huge learning curve for an already-skilled artist.
And yes Lathan, ive actually made quite a few buttons myself as 3D boxes with fancy lighting and rendered them in top-down mode :D