I have a custom template for a woocommerce category page to only display the categories. I have got the system to get a list of the child categories (by using get_term_chi
Had a similar setup but when I used what you did I didnt actually get the thumbnail file I got the full image file so instead I used this: wp_get_attachment_thumb_url so that my output url would be "../my-images"/image-150x150.jpg" and actually got it to pull the thumbnail image, just incase anyone runs into a similar situation..