Not that it makes any difference, but if they were going for a villain on each Blu-ray cover, it should have gone:
Maul
Dooku
Grevious
Tarkin
Vader
Emperor
Boba Fett had even less of a role than Grevious in Empire, so it doesn't make any sense to put him on the cover. (If you did, however, you should put him on Empire then replace Grevious with Vader)