What I did to retrieve my cover art,
In the reader there is a directory name ".kobo" and inside I found a sub name "tmp" and "images". In "tmp" one directory named "images". I copied all those files into the directory "images" which is in the ".kobo" directory. I use the reset and restarted the Kobo and all the cover art were present.
This worked for me, I hope this will work for you. You should backup the Kobo before doing anything. There is no guaranty that this will work.
Have a good day.