Three things that have become easier to customize are the Boot Splash screen, the Login screen and GRUB, the place your computer initially goes and where, if you are booting between multiple operating systems, you go to choose which operating system you want to run.
In later versions of Mint KDE, and im most recent ubuntu based Linux distributions, the easiest way to change this is to install grub-customizer.
Grub Customizer is available via PPA, just add the ppa to your MINT system, update the local repository index and install the grub-customizer package.
Open a terminal and copy and paste these lines (You WILL be asked for your root password):
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer
Ok, NOW to the reason for this post.
A lot of people use Grub customizer and are disappointed when they add a wallpaper image and it doesn't appear.
Google the problem and you will see all sorts of answers including:
Grub can be fussy about whether the image is jpeg or png.
Grub can be fussy about the size of images matching the screen.
Grub can completely die if you mess with the default font size or style and it overlaps the screen. (I would recommend sticking with the default 10pt text).
Grub Background Image MUST be set to 'transparency' if you want your image to show behind the text.
However one of the easiest things to check if your background image does not come up in Grub, is file names. DO NOT HAVE SPACES in your file names, or in the names of the folders where grub has to look for the image!
It is such a simple thing to forget, and most of us do. Create a separate folder for wallpapers and background images. Make sure that folder and its sub-folders do not have spaces in the names. replace the spaces with something like a hyphen, or just remove the space. Do the same with any images.
Freds-Christmas-Party.png is ok. Freds Christmas Party.png is NOT ok.
Also, I would be wary of punctuation in the file names. Do NOT have Fred's, because grub just might think that is part of a command, and fail.
Ok. So if you use Grub Customizer, and your background image is not showing, look for spaces in file and folder names first. Then, if that doesn't work, start looking at the other possibilities.