I'd bet that isn't true and that plenty of artists can and do peruse places like deviantart and quickly improve upon someone else's images.

But design for a GUI is more like industrial design. Getting a designer to help improve KDE's look-and-feel would be like squatters in a dilapidated hotel convincing an interior decorator to spruce the place up for fun.

Actually, that sounds like it would be a blast. Perhaps open source people are simply terrible at marketing.