How to make random on load page.

You would use a WP_Query to call only the Featured Image of all the posts and in the wp_query $args, the “orderby” would be ‘rand’ (random order), similar to this:

<?php
$args = array(
	'posts_per_page' => 4,
	'orderby' => 'rand'
);
?>

<?php $the_query = new WP_Query( $args ); ?>
<?php if ( $the_query->have_posts() ) : while ( $the_query->have_posts() ) : $the_query->the_post(); ?>

	<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
		<?php if ( has_post_thumbnail() ) {
			the_post_thumbnail();
		} ?>
	</div>

<?php endwhile;
wp_reset_query();
endif; ?>