First step, we need to prepare the picture that we want to use for GRUB image. Follow to default picture is should scale at 533 pixel width, 400 pixel height and use PNG format. We should named it sabily.png. After you had prepared it, we can continue to next step.
Now we must have full access authority for our file in File System partition. In order to do that, we need to install "nautilus-gksu". We can install it by run installation command on terminal.
"sudo apt-get install nautilus-gksu"
After installation completed, restart our Sabily system.
Now open our GRUB folder where GRUB image placed there. It locate at usr/share/images/grub. Right click on grub folder and select open as administrator.
Overwrite when asked.
Restart your sabily system. You should see new grub image that you change appears.
Here example of my GRUB image.