You might try
Kindle Comic Converter There is an Aura One example epub you can download. You can read .cbz files in KOReader, so you might want to try one that way and compare the results.
Luck;
Ken
P.S.; The Aura One downloaded as a .kepub.epub and looks fine on my AuraHD, in KOReader. The thumbnail looks ok in its History listing (I have the History set to use cover thumbnails)