Try Following Steps
1. open Terminal
2. type sudo apt-get install grub2-splashimages
3 . after installing the above check folder /usr/share/images/grub/
4. you can add images to this folder by pasting images to this folder
for changing the permissions of the folder
5. after this type sudo gedit /etc/grub.d/05_debian_theme in terminal
6. find the line set_background_image "/usr/share/images/desktop-base/desktop-grub.png"
you can use Cntrl + F to find the line
7. change the desktop-grub.png to the filename you like and save
8. then type update-grub in the terminal
9. restart the system ,now you can see your custamised grub background
for changing again background image edit the file /boot/grub/grub.cfg
1. open Terminal
2. type sudo apt-get install grub2-splashimages
for changing the permissions of the folder
5. after this type sudo gedit /etc/grub.d/05_debian_theme in terminal
you can use Cntrl + F to find the line
8. then type update-grub in the terminal
9. restart the system ,now you can see your custamised grub background
for changing again background image edit the file /boot/grub/grub.cfg
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