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add_filter('manage_posts_columns', 'featured_image_add_column');
add_filter('manage_pages_columns', 'featured_image_add_column');

function featured_image_add_column($columns) 
{
	$columns['featured_image'] = 'Featured Image';
	return $columns;
}

add_action('manage_posts_custom_column', 'featured_image_render_post_columns', 10, 2);

function featured_image_render_post_columns($column_name, $id) 
{
	if($column_name == 'featured_image')
	{
		$thumb = get_the_post_thumbnail( $id, array(50,50) );
		if($thumb) { echo $thumb; }
	}
}