While some people are content to take a screen grab in the 640x480 world of the fullscreen iTunes Visualizer, I choose this:
Set your monitor resolution to 1024x768 (this is a standard size these days)
-Open iTunes
-Start a song
-Goto 'visualizer' deselect 'fullscreen'
-Resize your iTunes window to it's Maximum size.
-then activate the visualizer.
-Press 'M' until 'Freeze current' appears.
-Use the 'q' 'w' 'a' 's' 'z' 'x' keys to choose your favorite visualizer arrangement.
-press space bar to pause.
The visualizer should come to a halt on the current setting (note: DO NOT PRESS ANY OF THE ARRANGEMENT KEYS AFTER YOU HAVE PAUSED THE MUSIC, if you do so, the current display will be lost and you will have one heck of a time getting the exact same thing back.)
Now, take a screen picture.
Open it in photoshop
-Crop an area within the visualizer that is 1024px by 768px.
(this is where you can get creative)
Choose a motion/radial Blur filter, I prefer the 'Zoom' filter, but it's up to you.
You can also adjust color/levels from here.
Now you should have a colorful, High resolution picture worthy of being used as a desktop background
