See video: https://youtu.be/hT9xU7jvwW4
When I import purchased tracks (AIFF + MP3) from Beatport or BPMSupreme, cover shows up in Serato. Those files come with ID3v2.4 metadata.
When I save them with MP3tag, the covers won't show up anymore in Serato.
When I alter/save them with iTunes, no such issue.
Only Serato seems to be affected.
Traktor & Rekordbox have no such issue after saving in ID3v2.4
Saving as ID3v2.3 no issues in Serato.
Perhaps this is something that can be fixed in MP3tag though, since when altering/saving with iTunes the files do show cover in Serato.