To create a custom icon, it must be in the vector SVG format. You can generate an SVG from Illustrator and just make sure all strokes are converted to paths.  The icons only use a single fill color so multi-color SVGs just need to be imported like an image into the media library. There are also a lot of free icons provided in SVG format like:

1). Minimize the SVG file size by dragging the SVG into this free online tool, the SVGOMG editor.

2). Turn on all of the settings except “Prettify code” and “Remove viewBox” and then click the “markup” tab at the top.  This will show you the tiny code to make the icon, select it, and copy.

3). Go into Showit and paste the code into the custom SVG box and you are good to go!