Low resolution album artwork? Why does Google Play Music and other music apps display album artwork with such terrible quality? This occurs even when high resolution images are embed into the audio files. How can we fix this?
There is a solution, but unfortunately it is a bit tedious. This issue occurs because Android caches the album art at a low resolution. It also generates file names for the thumbnails in a weird manner (apparently by the value of the system clock the first time the album appears). This means that the fix will be manual unless you employ some computer vision software. 


Navigate to Internal Storage\Android\data\com.android.providers.media and copy the albumthumbs folder to some local directory. 
You'll see files such as 1345139729376, 1345139731350, etc in albumthumbs. These files have no extension but are actually just jpegs. You can just edit these files if your image editing software supports it, but we could add .jpg to make it easier. In Windows this could be done via the command-line with ren * *.jpg. 
Now if you're on Windows you'll probably see image thumbnails instead of the generic file icon and it will be easy to edit the images. I'm sorry if you have a lot of albums, but since the file names are as good as random we need to actually look at each image and replace it with its high resolution counterpart. 
If you changed the file extension we can change it all back with ren * *. (note the period at the end). We can then paste our modified album art back into the original albumthumbs folder on the device. 


The result: