Kindle (stock firmware)
Stock Kindle firmware does not support custom screensavers without a jailbreak, and some models show ads or the book cover instead. The simplest route is to install KOReader (where your model supports it) and use the plugin above, or use the Kindle’s own “show book cover” option where available.
If your Kindle is already jailbroken with a screensaver hack, download at your model’s preset and follow that hack’s documentation for where images go (commonly a linkss/screensavers folder, PNG, grayscale).