Override the WooCommerce subcategory thumbnail to fallback to a product image or the standard fallback image

wc-category-image-fallback.php

<?php

/*
Plugin Name: WC Category Image Fallback
Description: WooCommerce Product Image Fallback for Categories.
Version: 1.0
Author: Andy Gee
*/


/**
 * Override the WooCommerce subcategory thumbnail to fallback to a product image or the standard fallback image.
 *
 * @param object $category The category object.
 * @return void
 */
function override_woocommerce_subcategory_thumbnail($category) {
  // Get the small thumbnail size for subcategories
  $small_thumbnail_size = apply_filters('subcategory_archive_thumbnail_size', 'woocommerce_thumbnail');

  // Get the dimensions of the small thumbnail size
  $dimensions = wc_get_image_size($small_thumbnail_size);

  // Get the thumbnail ID of the category
  $thumbnail_id = get_term_meta($category->term_id, 'thumbnail_id', true);

  // Initialize the image variable
  $image = null;

  // If the category has a thumbnail
  if ($thumbnail_id) {
    // Get the image source and set it to the image variable
    $image = wp_get_attachment_image_src($thumbnail_id, $small_thumbnail_size);
    $image = $image[0];

    // Get the image srcset and sizes attributes if the corresponding functions exist
    $image_srcset = function_exists('wp_get_attachment_image_srcset') ? wp_get_attachment_image_srcset($thumbnail_id, $small_thumbnail_size) : false;
    $image_sizes = function_exists('wp_get_attachment_image_sizes') ? wp_get_attachment_image_sizes($thumbnail_id, $small_thumbnail_size) : false;
  } else {
    // Get all the products in the category
    $products = wc_get_products([
      'category' => get_term($category->term_id, 'product_cat')->slug
    ]);

    // If there are products in the category
    if (!empty($products)) {
      // Loop through the products
      foreach ($products as $product) {
        // Get the image ID of the product
        $image_id = $product->get_image_id();

        // If the product has an image ID
        if ($image_id) {
          // Get the image source and set it to the image variable
          $image = wp_get_attachment_image_src($image_id, $small_thumbnail_size);
          $image = $image[0];

          // Get the image srcset and sizes attributes if the corresponding functions exist
          $image_srcset = function_exists('wp_get_attachment_image_srcset') ? wp_get_attachment_image_srcset($image_id, $small_thumbnail_size) : false;
          $image_sizes = function_exists('wp_get_attachment_image_sizes') ? wp_get_attachment_image_sizes($image_id, $small_thumbnail_size) : false;
          break;
        }
      }
    }

    // If no image is found, use the WooCommerce placeholder image
    if (!$image) {
      $image = wc_placeholder_img_src();
      $image_srcset = false;
      $image_sizes = false;
    }
  }

  // If an image is found
  if ($image) {
    // Replace spaces in the image URL with '%20'
    $image = str_replace(' ', '%20', $image);

    // Output the image with the appropriate attributes
    if ($image_srcset && $image_sizes) {
      echo '<img src="' . esc_url($image) . '" alt="' . esc_attr($category->name) . '" width="' . esc_attr($dimensions['width']) . '" height="' . esc_attr($dimensions['height']) . '" srcset="' . esc_attr($image_srcset) . '" sizes="' . esc_attr($image_sizes) . '" />';
    } else {
      echo '<img src="' . esc_url($image) . '" alt="' . esc_attr($category->name) . '" width="' . esc_attr($dimensions['width']) . '" height="' . esc_attr($dimensions['height']) . '" />';
    }
  }
}

/**
 * WooCommerce category thumbnails don't have a filter.
 * Remove the original action and add a custom function.
 */
remove_action('woocommerce_before_subcategory_title', 'woocommerce_subcategory_thumbnail', 10);
add_action('woocommerce_before_subcategory_title', 'override_woocommerce_subcategory_thumbnail', 10);