ok....
step 1: open 'grab'
step 2: open system prefs and go into your screen savers
step 3: in grab get it to capture a 'timed window' this will cause it to take a picture of your screen in 10 seconds time.
step 4: run the 'itunes artwork' screen saver
step 5: so now you have a capture of all your album art work ready to use as your wallpaper! you may want to crop it abit in preview or PS to get it prefect.
please enjoy!