This tool provides a simple way of doing that. The same goes for the color scheme, panel background, task items, etc. Hopefully it'll encourage artwork on individual items as well (users can "Select item from file...").
This is admittedly for the "power-tweakers" (I confess that I can't keep my desktop wallpaper the same for more than a couple weeks!) who want to fiddle a bit more than making a wholesale change of the entire desktop theme as is currently allowed with the "Desktop Settings" dialog.
Currently the tool can save and remove themes as well. I hope to add functionality for exporting the theme to a zip file and selecting from existing color schemes created by the colors KControl module.
(Note that this is under review right now, so there may be changes based on the feedback provided by the review).