Hi there.
Am using a PW2 on 5.6.5 firmware. I have jailbroken my kindle and got a system dump. While I check .jar assets, pngs, jpgs, I couldn't find one used as a default cover image for no-image books on my homescreen.(ss01.png is too large!)
update: Thanks to the modern firmware 5.7.2. No more annoying labels.

I found all the covers in
app_utilities-assets.jar.
The last problem is ,How do I properly replace images or even js inside the jar and resign the jar to be used by kindle? Do I need to generate my own keys for that?