The following is written to add a background image to GRUB2 in Linux Mint. This will work with Mint 22.1, it will also work with Ubuntu, and Debian. Your mileage may vary with other distros.
Make a copy of your GRUB.
sudo cp /etc/default/grub /etc/default/grub.bak
Edit your GRUB as sudo.
sudo gedit /etc/default/grub
Look for #GRUB_GFXMODE
Uncomment and enter a value that uses the ratio of your monitor such as 16:9 values could be 2560×1440, 1920×1080, 1600×900 and so on.
GRUB_GFXMODE=1920x1080
ADD a NEW LINE below that line like so:
GRUB_BACKGROUND=/home/yourusername/Pictures/awesomesauce.jpg
SAVE the file.
Now update grub with this command:
sudo update-grub
Enjoy the new background image of your GRUB2 menu….