Do you have access to a computer in your local library?
Also, you can actually access your Google Play Music library on your mobile Chrome browser, and I believe you can change album art that way. But it's a little tedious. Go to music.google.com on your mobile Chrome browser, then tap Menu and check on the Request Desktop Site option. Now pick an album, and tap the 3 vertical dot menu button at the upper right. Select Edit Album Info, tap the Change option on the album art image, and select the image you want to replace it with from your image list.