The below will change the grub splash image at boot time. Requires Imagemagick. Do it at your own risk!
Prepare your picture in any format.
sudo cp /boot/grub/splash.xpm.gz ./splash.xpm.gz.bak convert -resize 640x480 -colors 14 YOUR_PICTURE.jpg splash.xpm gzip splash.xpm sudo cp splash.xpm.gz /boot/grub/
