/home/pi/RetroPie/splashscreens
you can also access this splashscreens folder from samba shares once its created. (Note you may need to install or restart samba shares from the setup script for the splashscreens folder to show up over samba shares) Make sure you create a new folder to place your splashscreen in- for example: /home/pi/RetroPie/splashscreens/video/yourvideo.mp4
/home/pi/RetroPie/splashscreens/retropie-extra
quiet splash plymouth.ignore-serial-consoles
on the same lineplymouth.enable=0
if you're using retropie