This was honestly never meant to be any kind of feature, but with how I originally setup the shortcode, it happened to allow for this; so I figured it wouldn’t do any harm to tell people this was possible on the live demo.
I suppose I could add some sort of feature to the new plugin that would allow you to set a folder of icons from your Child theme if you wanted, but I’m thinking it would be a little overkill. It would seem strange to me to have that because all the shortcode does is output the image.
So, you could just upload your icon through WordPress and put it in like this:
Seems kind of overkill to do the extra checking and processing of allowing a feature like that when you could just insert your custom icon like that without any shortcode processing.