the splash screen is tricky lol your device is made to only be flashed once and never again lol so yes the images are on the boot.img, thats how they are installed to the device, but once they are there they will always be there (same as your charging images when phone is powered off).. even if you unpack boot.img and change what's in it.. you will not have a resulted change because the images are stored on the device already and will not overwrite..
the only option i can think of.. (
not proven anywhere and im pulling it out my ass so dont try this lol) but my thought would be to adb push the new images to the bootimage directory on your device

if you open astro or any type of file manager and get yourself to the highest root directory you will see some folders for boot image and charger image.
but it is extremely risky because we dont have access to these folders at all!!! so if you push any type of image/file that the device don't like.. you could find yourself in some serious doo doo lol