PostGrid: How to use featured image instead of permalink on postgrid page

gistfile1.txt

/**
 * Filter the permalink for a postgrid posts if it is a custom post type
 *
 * @since 1.5.0
 *
 * @param string  $permalink    The permalink for the post in question
 * @param int     $grid_post_id  The post ID in question.
 * @param int     $brick_id      The Brick ID containing the post grid
 */
function my_custom_permalinks_for_postgrid( $permalink, $grid_post_id, $brick_id ){
		
	global $wp_query;
	
	//If this page is the page with the postgrid on it
	if ( $brick_id == '4188' ){
		
		
		$featured_image = mp_core_the_featured_image( $grid_post_id, "1000" );
		
		//If there is a featured image for this post
		if ( !empty( $featured_image ) ){
			
			//Use the post's featured image as the permalink at 1000pixels width
			return $featured_image;
		}
		//If this post doesn't have a featured image
		else{
			
			//Use the normal permalink
			return $permalink;
		
		}
		
	}
	//Otherwise, if it is not the page with the postgrid on it
	else{
		
		//Use the normal permalink
		return $permalink;
		
	} 
	
}
add_filter( 'mp_stacks_postgrid_grid_post_permalink', 'my_custom_permalinks_for_postgrid', 10, 3 );