it needs to be done on a computer... its simple, just not streamlined.
if you put the right info in, it will appear automatically.
if you want to manually change it do the following:
1. download your album art to your PC. i use
Album Art Exchange - High Quality Album Art / CD Cover Art: Home and have good experiences with it. really nice high quality (i use 600x600 or higher and they look great on the phone) and they have most album covers you could want.
2. open up google music
2. go to the "album" section
3. hover the mouse over the album you want to change
you should see this, click on the little triangle hilighted in the red square
4. choose "edit album info"
5. hover over the headphone picture (or the incorrect album art pic) on the right of the pop-up and click the "change" button..
6. it will open up a file explorer...navigate to the saved album art image, and choose it.
7. choose save
and you are done.
thats how i do it. it took me a while to get them all when i first uploaded my 10K songs...but now that i have them up to date, anything that i add, literally takes me about 2 minutes to add the album art.