You don't have to use Grub Customizer to add a picture of your choice to your Grub Menu.
Instead you can edit a picture of your choice with the .png extention and place it in your /boot/grub directory.
In this screenshot you'll see where I placed the Ocean.png picture to the /boot/grub directory.
Once I moved that picture to the /boot/grub directory I right clicked on the directory and choose "open as root" and changed the Ocean.jpeg to Ocean.png.
Then I opened the /etc/default/grub file and added this string to the file.
GRUB_BACKGROUND=/boot/grub/Ocean.png
After you add that line to your /etc/default/grub file update Grub like so:
#sudo update-grub
I was looking for Grub Customizer in the Software Manager for my Linux Mint 21 installation and couldn't find it. So after a discussion with a few of my Linux friends I found out that Grub Customizer made a mess for them and they had to perform a fresh installtion of their distro's.
In this article I found out why one shouldn't use Grub Customizer here: