add-category-thumbnail-to-pdp.php

<?php

/**
 * @snippet       Utilizes the Product Category Thumbnail to apply itself in place of the PDP's Main image on all products in that category.
 * @author        Serafin Tech
 * @compatible    WooCommerce 9+
 * @website       https://serafintech.io
 */



add_filter('woocommerce_product_get_image', 'use_category_thumbnail_if_no_product_image', 10, 3);
function use_category_thumbnail_if_no_product_image($image, $product, $size) {
    if (!has_post_thumbnail($product->get_id())) {
        $terms = get_the_terms($product->get_id(), 'product_cat');

        if ($terms && !is_wp_error($terms)) {
            $category = $terms[0];

            $thumbnail_id = get_term_meta($category->term_id, 'thumbnail_id', true);

            if ($thumbnail_id) {
                $image = wp_get_attachment_image($thumbnail_id, $size);
            }
        }
    }

    return $image;
}

add_filter('woocommerce_single_product_image_thumbnail_html', 'replace_single_product_image_with_category_thumbnail', 10, 2);

function replace_single_product_image_with_category_thumbnail($html, $post_id) {
    global $product;

    if (!has_post_thumbnail($post_id)) {
        $terms = get_the_terms($post_id, 'product_cat');

        if ($terms && !is_wp_error($terms)) {
            $category = $terms[0];

            $thumbnail_id = get_term_meta($category->term_id, 'thumbnail_id', true);

            if ($thumbnail_id) {
                $image_url = wp_get_attachment_image_url($thumbnail_id, 'full');
                
                if ($image_url) {
                    return '<img src="' . esc_url($image_url) . '" alt="' . esc_attr($category->name) . '">';
                }
            }
        }
    }

    return $html; 
}

This is for those who want the product category thumbnail to apply to all PDPs in said category. Use at your own risk. Tested up to 6.6.2 core and WooCommerce 9.3 Code should be added to your functions.php file of your child theme