WooCommerce - Just one thumbnail in product page.

functions.php

function custom_woocommerce_show_product_thumbnails() {
	global $product;

	$attachment_ids = $product->get_gallery_attachment_ids();

	if ( ! empty( $attachment_ids ) ) {
		$attachment_id = current( $attachment_ids ); // pega o primeiro

		$image_link = wp_get_attachment_url( $attachment_id );

		if ( ! $image_link ) {
			return;
		}

		$image       = wp_get_attachment_image( $attachment_id, apply_filters( 'single_product_small_thumbnail_size', 'shop_thumbnail' ) );
		$image_title = esc_attr( get_the_title( $attachment_id ) );

		echo sprintf( '<a href="%s" title="%s">%s</a>', $image_link, $image_title, $image );
	}
}

add_action( 'woocommerce_product_thumbnails', 'custom_woocommerce_show_product_thumbnails', 20 );