Grub Customizer is a nice program to make adjustment to your Grub Menu.
There are a few things you can do in Grub Customizer however; I will only be covering how to change your background image for your Grub Menu.
-- I'm also including information on a program called XnConvert that converts the size of a picture which you may need.--
When you open Grub Customizer you'll see List Configuration, General Settings and Appearance Settings.
There's two way to add the picture of your liking to the Grub Menu.
You can click on the plus button next to the Themes drop down menu and go through the path to the directory where your picture is. Or you can click on the small picture icon at the bottom of the list on the left hand side of the menu under background image.
Before this works (and I had to restart my pc like 5 times) to get it right, you have to change the picture image that you want to .png. And make sure that the custom resolution box is unchecked. Then click on save in the upper left hand side of the application. The configuration regenerates and upon reboot you should be greeted with your new picture image and your operating systems listed in your Grub Menu at boot up.
To ensure that things are right you can check the /etc/default/grub file in your terminal. To do that open your terminal and run cat /etc/default/grub and press Enter. The last few lines should reflect what the Grub Customizer program arranged for you.
Here's mine:
export GRUB_COLOR_NORMAL="light-gray/black"
export GRUB_COLOR_HIGHLIGHT="light-blue/black"
export GRUB_MENU_PICTURE="/home/alex/Pictures/36548_result.png"
To change your picture image size you may need a program to resize it.
I use; under Debian, a program called "XnConvert" It's very easy to use.
It's available for 32-bit and 64-bit Linux Debian systems. Check your package manager or Software Center first, as it may already be there so you don't have to download it and install it--
Here's the link.
Here's other programs to convert images for Linux:
https://alternativeto.net/category/phots-and-graphics/image-converter/?platform=linux
When XnConvert opens just click on "Add files" and select the picture image you want to resize. Then click on "Output" and under the "Format" heading in the drop down menu scroll until you see png and select png. Then click convert.
That's pretty much it.
Once that background picture image is converted to png Grub Customizer should recognize it after you save it and upon reboot you will be pleased with your new background image.
Enjoy the holiday!