Hacks DIY Photoframe using a kindle

[dushu]

Hi everyone - so I have an old kindle that fell down and the shell broke. It's perfectly functional otherwise and works like a charm. However, because of the broken shell, I don't use it. Thought of converting it to a simple photo frame - but all the tutorials I've seen involve jailbreaking it. Unfortunately, the kindle firmware is already updated to the latest, so cannot jailbreak it. Came up with another idea:
Use the kindle feature of showing the ebook cover as the lock-screen
Using a raspberry pi, mount the kindle
Then I would change the ebook file with a new cover (can use calibre command line for this)
Re-upload the file
eject the kindle
Unfortunately, even though I change the file, and can see the new cover, the kindle keeps showing the old cover. I've deleted the thumbnails, I've deleted the system folder, but the old cover remains.
I know this is not a common use-case, but any help would be appreciated. Thanks