/**
 * Use as product image url the value of a custom meta field
 * NOTE: to be used with the custom theme templates
 * <your_tema>/woocommerce/single-produict/product-image.php
 * <your_tema>/woocommerce/single-produict/product-thumbnails.php
 */
function custom_woocommerce_thumbnail() {
	global $product;

	$id = get_the_ID();
	$title = get_the_title();
	$size = 'shop_catalog';

	$post_thumbnail = get_the_post_thumbnail( $id , $size );

	if ($post_thumbnail === '') {
		echo "<div class='thumbnail_container'>";

		//$main_image = get_post_meta($id, 'main_image', true);
		$main_thumb = get_post_meta($id, 'main_thumb', true);

		if ($main_thumb === '') {
			$post_thumbnail = '<img width="450" height="450" alt="' . $title . '" class="attachment-shop_catalog wp-post-image" src="' . wc_placeholder_img_src() . '">';

		} else {
			$post_thumbnail = '<img width="450" height="450" alt="' . $title . '" class="attachment-shop_catalog wp-post-image" src="' . $main_thumb . '">';
		}
		echo $post_thumbnail;
		echo "</div>";
	}
}
remove_action( 'woocommerce_before_shop_loop_item_title', 'woocommerce_template_loop_product_thumbnail', 10);
add_action( 'woocommerce_before_shop_loop_item_title', 'custom_woocommerce_thumbnail', 10);

/**
 * Add the column for the product image and remove the default column from products list
 */
function custom_admin_products_image_column( $columns ){
	//unset( $columns['thumb'] );

	$ordered_columns = array();
	foreach ($columns as $key => $column) {
		$ordered_columns[$key] = $column;
		if ($key == 'thumb') {
			$ordered_columns['custom-image'] = 'Image';
		}
	}
	return $ordered_columns;
}
add_filter( 'manage_edit-product_columns', 'custom_admin_products_image_column' );

function boralevi_admin_products_image_column_content( $column, $product_id ){
	if ( $column == 'custom-image' ) {
		//$product = wc_get_product( $product_id );
		$main_thumb = get_post_meta($product_id, 'main_thumb', true);

		if ($main_thumb == '') {
			$main_thumb = wc_placeholder_img_src();
		} 
		echo '<img src="' . $main_thumb . '" class="woocommerce-placeholder wp-post-image" alt="Placeholder" loading="lazy" width="80" height="80">';
	}
}
add_action( 'manage_product_posts_custom_column', 'custom_admin_products_image_column_content', 10, 2 );

<?php
/**
 * Single Product Image
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product/product-image.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see     https://docs.woocommerce.com/document/template-structure/
 * @package WooCommerce/Templates
 * @version 3.5.1
 */

defined( 'ABSPATH' ) || exit;

require_once 'Mobile_Detect.php';

// Note: `wc_get_gallery_image_html` was added in WC 3.3.2 and did not exist prior. This check protects against theme overrides being used on older versions of WC.
if ( ! function_exists( 'wc_get_gallery_image_html' ) ) {
	return;
}

global $product;

$product_id    = $product->get_id();
$product_title = $product->get_title();

$main_image = get_post_meta($product_id, 'main_image', true);
$main_thumb = get_post_meta($product_id, 'main_thumb', true);

$html = '';

$columns           = apply_filters( 'woocommerce_product_thumbnails_columns', 4 );
$post_thumbnail_id = $product->get_image_id();
$wrapper_classes   = apply_filters(
	'woocommerce_single_product_image_gallery_classes',
	array(
		'woocommerce-product-gallery',
		'woocommerce-product-gallery--' . ( $product->get_image_id() ? 'with-images' : 'without-images' ),
		'woocommerce-product-gallery--columns-' . absint( $columns ),
		'images',
	)
);
?>
<div class="<?php echo esc_attr( implode( ' ', array_map( 'sanitize_html_class', $wrapper_classes ) ) ); ?>" data-columns="<?php echo esc_attr( $columns ); ?>" style="opacity: 0; transition: opacity .25s ease-in-out;">
	<figure class="woocommerce-product-gallery__wrapper">
		<?php
		if ($main_image != '') {

			$detect = new Mobile_Detect;

			if ($detect->isMobile()) {
				echo sprintf('<img style="padding: 0;" src="%s" class="woocommerce-main-image" title="%s">', $main_thumb, $product_title);
			} else {
				echo sprintf('<img style="padding: 0;" id="elevatezoom-target" src="%s" data-zoom-image="%s" class="woocommerce-main-image" title="%s">', $main_thumb, $main_image, $product_title);

				echo '<p class="spiegazione-zoom">' . __('Use the mouse to view carpet details.', 'boralevi') . '</p>';
				echo '<p class="spiegazione-zoom">' . __('Use the mouse wheel to zoom the detail.', 'boralevi') . '</p>';
			}
			//echo sprintf( '<a href="%s" title="%s" data-rel="prettyPhoto">%s</a>', $main_image, $product_title, __('Click here to view full image', 'boralevi'));

		} else{
			if ( $product->get_image_id() ) {
				$html = wc_get_gallery_image_html( $post_thumbnail_id, true );
			} else {
				$html  = '<div class="woocommerce-product-gallery__image--placeholder">';
				$html .= sprintf( '<img src="%s" alt="%s" class="wp-post-image" />', esc_url( wc_placeholder_img_src( 'woocommerce_single' ) ), esc_html__( 'Awaiting product image', 'woocommerce' ) );
				$html .= '</div>';
			}
			echo apply_filters( 'woocommerce_single_product_image_thumbnail_html', $html, $post_thumbnail_id ); // phpcs:disable WordPress.XSS.EscapeOutput.OutputNotEscaped
		}

		do_action( 'woocommerce_product_thumbnails' );
		?>
	</figure>
</div>

<?php
/**
 * Single Product Thumbnails
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/single-product/product-thumbnails.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see         https://docs.woocommerce.com/document/template-structure/
 * @package     WooCommerce/Templates
 * @version     3.5.1
 */

defined( 'ABSPATH' ) || exit;

global $product;

$temp_gallery_images = get_post_meta($product->get_id(), 'gallery_images', true);
$temp_gallery_thumbs = get_post_meta($product->get_id(), 'gallery_thumbs', true);

if ($temp_gallery_images != '') {
	$gallery_images = explode(',', $temp_gallery_images);
} else {
	$gallery_images = array();
}
if ($temp_gallery_thumbs != '') {
	$gallery_thumbs = explode(',', $temp_gallery_thumbs);
} else {
	$gallery_thumbs = array();
}

echo '<div class="thumbnails">';

$main_image = get_post_meta($product->get_id(), 'main_image', true);
$main_thumb = get_post_meta($product->get_id(), 'main_thumb', true);

if ($main_image != "") {
	echo template_to_html( $main_image, $main_thumb, $product->get_title());
} 

for ($i = 0; $i < count($gallery_images); $i++) {
	echo template_to_html( $gallery_images[$i], $gallery_thumbs[$i], $product->get_title() );
}

echo '</div>';

function template_to_html($img, $thumb, $title) {
	$template = '<a href="%s" class="" title="%s" rel="product_images[grouped]">
		<img src="%s" class="attachment-shop_thumbnail size-shop_thumbnail" alt="%s" width="100" height="100">
		<span class="image-overlay overlay-type-image" style="left: -1px; top: 4px; overflow: hidden; display: block; height: 120px; width: 130px;">
			<span class="image-overlay-inside"></span>
		</span>
	</a>';
	
	return sprintf($template, $img, $title, $thumb, $title);
}