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If you create an iTunes account (it's free, but you need to have a debit/credit card). Then iTunes will automatically download album artwork as long as you have the album name in iTunes.

There is a 3rd party program called iTunes Keys (which only works on Window's) which is free and downloads any album artwork.
 
Search www.dashboardwidgets.com

There is a widget called Amazon Album Art that allows you to get the artwork of the album you're listening to and adds it to iTunes.

Or... try out CoverScout (an app to download all missing artwork), just google it :)

Hope it helps. cheers.
 
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