Actually all you have to do is save your art on your SD card, by going to the Preferences screen in the app. the Preferences button is located in the lower right hand side of the screen. there is an option to save your artwork onto the SD card. there's also an option to name what file to place the album art in. Save album art on to your SD card, then go down to the option of re scanning your SD card. by pressing this optio playern the SD card will be re scanned in your album art will be yonr Google Music player
Unfortunately "Rescan Media" states Scan not available on KitKat and above. Otherwise, after selecting preference Auto-save to SDCard, perhaps the rescan media would have allowed the album art to be displayed in Google Play. Hopefully some day.