As far I see JetPack’s U-boot does not have a graphic driver for DSI panels.
So, it is not possible to show a picture during startup without writing a special code for it.
But you have a chance to choose which display becomes a system console.
Just look at the /boot/extlinux/extlinux.conf
fbcon and console options point kernel to use appropriate display devices:
APPEND fbcon=map:0 console=tty0 console=ttyS0,115200n8 ...