Note: This will not change the screen without text(please wait etc..)/loading bar.
Prerequisites:
sftp(needs ssh). On the wiki.
Let's get started!
First, get a grayscale 600x800 image to replace the old one. I chose:
1)navigate to /usr/share/blanket
2)rename bg_xsmall_defaultbg.png to bg_xsmall_defaultbg-bak.png
3)rename your image to bg_xsmall_defaultbg.png and put a copy there
4)enter folder splash and repeat steps 2-3
5)go to parent dir and then enter shutdown
6)rename bg_xsmall_reboot.png to bg_xsmall_reboot-bak.png
7)rename your image bg_xsmall_reboot.png and put a copy there
8)eject
You're done.