I have the same question. Images are in folder (folder.jpg) and there are images in the files themselves, too. But nevertheless with MPD (Kodi on raspberry) using glider there are no images shown at all.
Thanks a lot for an answer.
P.S. … and after changing some folder- and file-names (Spaces substituted with Underscores) these albums/titles can’t be played, long waiting, nothing happens when clicking on titles – even after syncing again or deleting and re-installing the glider app again. Playing these albums in Kodi directly is no problem at all.