Grub Customizer is good gui tool to configure grub including changing the background with few clicks.
Unfortunately, it's not available using the official repositories provided by Ubuntu. So, you'll have to add its ppa to be able to install it.
1- Open a terminal window and paste
2- Hit enter to accept adding the new ppa
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
3- Update the repositories' lists and install Grub Customizer
4- Launch the program normally by searching for it using the Dash Home menu
sudo apt-get update && sudo apt-get install grub-customizer
5- The program will scan your grub entries once launched in few seconds. Then you can go to Preferences>>Appearance tab>>Background Image
Note to the original poster of the thread: It seems that you are somehow knowledgeable and prefer the command line way. Surely, we won't go into the usual debate of gui vs cli, but my post might be useful to beginners (especially that you've posted in Absolute Beginner Talk) or those who prefer a simple gui to do simple stuff.