Add featured image column to WordPress admin post and page lists

functions.php

add_image_size( 'bestwebsite-admin-post-featured-image', 120, 120, false );
 
// Add the posts and pages columns filter. They can both use the same function.
add_filter('manage_posts_columns', 'bestwebsite_add_post_admin_thumbnail_column', 2);
add_filter('manage_pages_columns', 'bestwebsite_add_post_admin_thumbnail_column', 2);
 
// Add the column
function bestwebsite_add_post_admin_thumbnail_column($bestwebsite_columns){
	$bestwebsite_columns['bestwebsite_thumb'] = __('Featured Image');
	return $bestwebsite_columns;
}
 
// Let's manage Post and Page Admin Panel Columns
add_action('manage_posts_custom_column', 'bestwebsite_show_post_thumbnail_column', 5, 2);
add_action('manage_pages_custom_column', 'bestwebsite_show_post_thumbnail_column', 5, 2);
 
// Here we are grabbing featured-thumbnail size post thumbnail and displaying it
function bestwebsite_show_post_thumbnail_column($bestwebsite_columns, $bestwebsite_id){
	switch($bestwebsite_columns){
		case 'bestwebsite_thumb':
		if( function_exists('the_post_thumbnail') )
			echo the_post_thumbnail( 'bestwebsite-admin-post-featured-image' );
		else
			echo 'hmm... your theme doesn\'t support featured image...';
		break;
	}
}