I've been using EasyTAG to add a photo to a mp3 file (album cover, really). However, for these files' logos in Nautilus, it only shows the default "mp3" icon, versus my other music, whose file icons are their album cover:
This is also shown in Amarok, which shows the group of songs' album cover, but not the picture for the song itself:
(album in lower left, song in center). Also, the files are purchased from the band's website, versus iTunes or Google Play, so that might be relevant. Thanks!