add_filter( 'aioseo_sitemap_images', 'aioseo_filter_sitemap_images', 10, 2 );

function aioseo_filter_sitemap_images( $images, $post ) {

	if ( ! function_exists( 'get_field' ) ) {

		return $images;

	}

	$customImage = get_field( 'custom_image', $post->ID );

	if ( ! empty( $customImage ) ) {

		$images[] = wp_get_attachment_image_url( $customImage['id'] );

	}

	return $images;

}