Prerequisite: bootloader unlocked, root access, S-OFF, the engineering HBOOT (I used this one)
Download the splash.img and place it in your working directory.
adb reboot bootloader
fastboot flash splash1 splash.img
Preview order is splash1, 2, 3, 4, 5:
Preview order is splash6, 7, Dre.jpg-splash.img
GIMP XCF to edit your own version here:
Also I do take requests. I'm working on a space themed one at the moment