You can use my python script to side load missing covers into your Paperwhite, Paperwhite2, Kindle Touch:
https://github.com/quiris11/ExtractCoverThumbs/releases
Code:
usage: ExtractCoverThumbs [-h] [-v] [-o] kindle_directory
positional arguments:
kindle_directory directory where is a Kindle Paperwhite mounted
optional arguments:
-h, --help show this help message and exit
-v, --verbose print more informations
-o, --overwrite overwrite thumbnails