In the category menu and sub collection list section, an icon image is used. The metafields option is used to makes it work. Follow the steps below to configure it.

Step 1 :

 From your shopify store admin > click "Settings" > Metafields and metaobjects > collections > Add definition

Step 2 : Fill in the details as shown in the screenshot below.

Step 3 : Navigate to the metafield section at the bottom of the collection page.

Step 4 : Click icon field to select image and save page.

If possible, use a transparent PNG file to make the section blend in with the background color.