There's info on how to change the power off and boot up images here (page 75):
http://download.pocketbook-int.com/6...23%28EN%29.pdf , but you're asking about the sleep image, right? (When the reader is paused, not turned off.)
I changed to Crystal theme (info in German here
https://www.mobileread.com/forums/sho...d.php?t=214445 , some related info here
https://www.mobileread.com/forums/sho...d.php?t=214763 ), and that changed the sleep image to a sleeping penguin, see the first thumbnail in the German thread. I've no idea how to change it to a custom image, but the second thread I linked to has some info on how to make your own themes.