How to set a background image on GRUB?

The Ubuntu wiki has a detailed guide. First, you need to make sure the image is in the right format: a 256-color non-indexed RGB JPEG. GRUB checks a number of different places for background settings, and it varies by version; here is the first place checked for two versions:

1.98

Open /etc/grub.d/05_debian_theme, look for the WALLPAPER= line, and set it to the correct path:

WALLPAPER=/path/to/your/bg.jpg

1.99 and up

Open /etc/default/grub and add a new GRUB_BACKGROUND line:

GRUB_BACKGROUND=/path/to/your/bg.jpg

Update the grub configuration file:

update-grub

Just put any image under /boot/grub/ and run

sudo update-grub