I don't do the book cover sleep screen, but some readers have the option to save the cover to some location.
I forget, but if you disable some Onyx stuff the location changes.
In any case, my sleep images are at:
Code:
/data/local/assets/images/shutdown.png
/data/local/assets/images/standby-1.png
/data/local/assets/images/standby-2.png
/data/local/assets/images/standby-3.png