The thing is, Kindles are very fussy about the formatting (dimensions, bit depth, etc) of the screensavers; the values tend to vary between Kindle models and there is a real risk your Kindle will end up in a boot loop if you mess it up.
Now, if you still want to go ahead despite the warning, the easiest way to do this on HF firmware is as follows:
- Inspect the file properties (e.g. right-click -> properties -> details on Windows) of your default kindle screensavers, which are located in folder /usr/share/blanket/screensavers on your Kindle
- Use this tool to convert your desired source images to the required screensaver format, by feeding the correct values identified in step 1
- Send the converted files to /usr/share/blanket/screensavers, either by copying them alongside the original screensavers, or by replacing them.
Good luck!