Oh right, I forgot to explain that. I hope this explains it enough, if not, please let me know and I'll create a simple example.
Create a subfolder in the folder containing the style you want to add images to, with the name of the style. Then inside that folder, place images. In the style editor for the keys, you can enter just the filename of that image, not the folders leading up to it, and it will try to load that image.
So, let's say I have one of your keyboards, it is stored in:
NohBoard\keyboards\Games\csgo_shift+caps\keyboard.json
I have created a keyboard specific version of your john style, so this file is called:
NohBoard\keyboards\Games\csgo_shift+caps\john.style
I have an image, called cat.jpg which I want to add to a button, then I place this image file in:
NohBoard\keyboards\Games\csgo_shift+caps\john\cat.jpg
I for whatever key I want to set the image, then fill in for background Image:
cat.jpg