I used a custom field for an image to categories. Using the custom field, I added an image for each subcategory. I'm using MB Views so that for a specific category archive, I can show the images for the subcategories. I would also like to pull in the subcategory title and the URLs for the subcategories.
Basically, you can use the field single_image and assign it to the taxonomy. Then in the View editor, click on the button Insert Field > tab Term > select Term info (name, URL ...) and Term Meta to show on the taxonomy page.
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