Remember that for album artwork to be made available to players other than itunes, it needs to be embedded into each individual mp3 file. Itunes does not do this by default, it uses a reference to a specially stored file in it's library. If you purchase mp3's from the iTunes store, Amazon or Play.com, the artwork should be embedded. We are talking about you burning your own CD's here I take it...
If (in iTunes) you select songs and manually drag artwork over to the artwork box, this will embed the artwork within mp3's, so, for example, if you then copy your files from your computer to your gs2, the artwork will be seen by all he music apps such as doubleTwist et al, without installing any other artwork grabbers.
If you have lots of albums in iTunes with artwork available to iTunes only (this happens even when you do the 'get album artwork' if it is not already done automatically) there is a neat way to force it to embed if you have a Mac. I'm sure there is also a way if you have a PC. For the Mac, download the action script called 'Embed Artwork'. A quick search on Google will point you in the right direction with instructions.
Once done, all you r mp3's will have the artwork available to any media player reading them...
Cheers
Steve