title_body: Is there a way to make third party book (non-amazon) covers appear as screensavers on Kindle?
upvoted_answer: After running some experiments I have finally managed to solve this issue for my Paperwhite4.
  For anyone having the same issue - follow this sequence:
  
  Take a side loaded document of any format
  Open it w Calibre
  Right click on it and press “Edit metadata”
  In a new window press the button “Download metadata” (it’s at the bottom of the screen)
  Follow thru with a dialog window
  Choose one of the book covers proposed by Calibre (do this step even in case you think your document already has a cover)
  Finish by pressing “Ok”
  Convert your document to mobi
  Hook up your kindle via USB
  Right click on the document in Calibre and choose “Send to device”
  Choose “Send specific format to”
  Choose “Main memory”
  Select mobi file from the list
  
  You are done. Your document’s cover will  display on the lock-screen.
  Essentially it appears that three things were key:
  
  downloading/ updating the file’s metadata and the cover via Calibre
  converting the file to mobi
  sending the file to kindle via USB (through Calibre as opposed to simple drag and drop using windows explorer)