Have the image you want on the USB or the Internal storage on the Unit
download this software - https://www.dropbox.com/s/0zg762khedx8pdw/boot%20screen%20changer.apk?dl=0
and run it on the unit, it should ask you which image you want to use - Find the image you want to
use as your boot up screen and it should save for the next boot up.