Here is a version of the offical Ubuntu default wallpaper (original by ngnr) for Kubuntu lovers.
I adjusted the colours in photoshop to blue, and replaced the Ubuntu logo with Kubuntu's. Sounds easy doesn't it, but it was trickier in practice without the original art layers to work with. The size, colour and positioning of the Kubuntu logo is very simpathetic to the original layout.
The original Ubuntu version was created by ngnr and released under the GPL.
My feeling is that Kubuntu doesn't look as good as Ubuntu. Maybe Kubuntu should always have the same artwork as Ubuntu, but altered to reflect Kubuntu's characteristic blue look, and showing the Kubuntu logo. Just an idea.