Hi. Firstly, thanks for replying.
I had already tried what you suggested. I found something weird.
I observed three books in my collection: The Three-Body Problem, Digital Fortress and The Hobbit. When I open all the three in 'Edit metadata' window in Calibre, all of them show a book cover on the left side. Like this:
https://i.imgur.com/hJRDfqW.png
But when them in Kindle, only the first two show a thumbnail cover and only these two work with the Screensaver hack (not working with The Hobbit). See this:
https://i.imgur.com/FxQHvUx.jpg
Why is that so?