The easiest way I have found to get the art is to do a Google images search. Go to the Google homepage and click images in the top left of the page this will take you to Google images search. Once there searching for the track/ album name and band name is the best way. You will get a bunch of thumbnails like in my second pic.
Select the thumbnail that best fits what you are after and you will be taken to the page that has the image on it. At the top of the page there will be a Google frame, click on "see full-size image". You want one that is roughly square and that is at least 200 by 200 to avoid blurring or distortion.
You should now have just the image you want in your browser. Right click on the image and select "save picture as" and save the image to the desktop.
All that remains to be done is to click and drag the image from the desktop and onto the generic CD image in your wmp11 library (you will notice it says paste art here). Alternatively if you have your browser and wmp11 open at the same time you can just click and drag the image straight from the browser to the library without saving to the desktop. (works with IE7).
That's your library art up to date and looking cool. Third pic. If you used the save the image to the desktop method you can delete the image now as it is no longer needed.