This [icon] shortcode is really just a way for us to provide you with some icons out-of-the box with the theme. If you’re wanting to add icons for use in this shortcode, or even override some of the theme’s default ones, it’s quite easy.
In your Child theme’s main directory create a folder called “icons” and put any PNG icons you want in there.
So, for example, when you use [icon image=”example”] in your site, first the theme will look for:
Your Child Theme/icons/example.png
And then if this doesn’t exist, it will look for “example.png” within the Parent theme.
Additionally, if you’re comfortable with basic HTML, you really don’t even need to use this shortcode. The equivalent would be just to do this with HTML: