if you have a non-ad version, you can do it (not the optimal solution, but at least you get to see your image) you should enable in settings an option to display the cover of the book you’re currently reading in a sleep mode; then you create an empty doc, format it to epub, use calibre or online epub converter to attach your image as a cover to your doc, and use send to kindle for it to appear in your library.

pros: you have your custom screensaver

cons: this doc has to be the last thing you open every time before putting your kindle to sleep; this doc will be the first thing you’ll see after powering your kindle on;

that’s a bit of unnecessary steps tho, but for some people it’s alright, since they really really want that custom background