I've done some research, and it seems there is a workaround for the android book cover problem I've mentioned before. The problem is that the device needs to be rooted, otherwise you don't have access to the required folder.
If you have a book with the ASIN "B000000001", then put a png formatted image in /data/data/com.amazon.kindle/files with the filename "cover_B000000001.png" and it will show up instead the downloaded garbage.
Hope it helps someone
.