
For Debian 9.x + MATE Desktop 1.16.x:

Open a terminal and type:

$ sudo apt-get install grub2-splashimages
$ sudo cp /usr/share/images/desktop-base/desktop-grub.png /home/YOURUSERNAME/ # Default GRUB image backup

Then copy the new desktop-grub.png image in /usr/share/images/desktop-base/

$ sudo update-grub
$ sudo update-grub2
$ sudo reboot

Done.

NOTE: For other distros (Ubuntu, Linux Mint, etc.) and Desktops Environments (GNOME, KDE, etc.), check their installation on internet.

