Thanks. With folder.jpg been put into each album, the album artwork can be displayed well (with my try, even 720x720 px is ok to display).
But I found another problem, it seems that VOX cannot recognize the song title and album name of a .APE file. (I checked the feature list of Kid3, https://kid3.sourceforge.io/, and it seems that Kid3 does NOT support .APE file. I'm not sure if it relates to VOX app or not)
Please see the attached files for my test: 1 for flac file, 1
for ape file.