Re: Google music album art
There's no way to do it from the Play Music app. Google assumes that you've either purchased music from the Play Store (which includes the correct album art) or uploaded MP3s with album art already embedded into the MP3 metadata.
If you are missing a lot of album art then I suggest that you use Media Monkey or another desktop music manager and embed all the album art in each MP3, then re-upload to Google Music.
Whenever I rip or download a track, the first thing I do is embed the appropriate album art and update the metadata with the right album, artist, etc. All of my music has album art embedded, so when I uploaded it to Google Music, I was all set.
There are apps in the Play Store that will add album art to MP3s, but they require you to first have the MP3 downloaded to your tablet. They can't update album art for MP3s already uploaded to Google Music. So you would have to delete the music from Google Music and re-upload the corrected copy.
Your other option might be to try to use the desktop Google Music site from your tablet. (Request Desktop Site option in the browser) I'm not sure how well that will work. Your browser might just take you to the Play Music app.