How do I display product category icons?

I’m using Woocommerce with Twenty Fifteen theme.

I want to display the product category icons.
I’ve already made thumbnails for each category and assigned to each category.

On other themes I have used, you can just pull in a block with the product categories.
As long as there is at least one product in a category, then the thumbnail will show.

I’m trying to use everything basic and restrict using extra plugins.

I was hoping Gutenberg would have a widget or something.
Alas no ??

How can I get what I want?

Thanks.