Looking around on the internet I found a few screenshots of customized GRUB installations with a background image set, is that possible? How can I do that?
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:
/etc/grub.d/05_debian_theme, look for the
WALLPAPER= line, and set it to the correct path:
1.99 and up
/etc/default/grub and add a new
Update the grub configuration file:
There is this program grub-customizer: https://answers.launchpad.net/grub-customizer/+faq/1397.
It can also change the fonts color, the fonts, edit all the entries and more.
Your distribuition may have the package so you don' t have to build it.
If you want only to change the image you can do as suggested by user126697:
Just put the image under /boot/grub/ and run