You will need Python itself and the Python Imaging Library. This is available as the python-imaging package on Ubuntu systems.
At its most basic, run ./wallpaper.py 1680 1050 to generate a 1680*1050 pixel wallpaper named wallpaper.jpg in the current working directory. ./wallpaper.py -h will list options; you can change the input directory, output filename, and cover size.
I recommend running it as a cron job with some given frequency, setting the KDE wallpaper to a slideshow (or some equivalent on your distro/OS), and pointing it at the directory you've set the output to in the cron command. This will automatically redraw your desktop with a new random set of album art on a regular basis.