<?php
/*
Plugin Name: Add Post Thumbnails to the WordPress Post and Page Management Screens
Plugin URI: http://www.ramoonus.nl/wordpress/add-thumbnails-to-post-list/
Description: This plugin adds a post thumbnails to the WordPress Post and Page Management Screens. The idea of this plugin comes from Sarah Gooding. 
Version: 1.0.0
Author: Ramoonus
Author URI: http://www.ramoonus.nl/
*/

//
// Copyright http://wordpress.stackexchange.com/questions/1567/best-collection-of-code-for-your-functions-php-file/6021#6021
// 

/****** Add Thumbnails in Manage Posts/Pages List ******/
if ( !function_exists('AddThumbColumn') && function_exists('add_theme_support') ) {
    // for post and page
    add_theme_support('post-thumbnails', array( 'post', 'page' ) );
    function AddThumbColumn($cols) {
        $cols['thumbnail'] = __('Thumbnail');
        return $cols;
    }
    function AddThumbValue($column_name, $post_id) {
            $width = (int) 60;
            $height = (int) 60;
            if ( 'thumbnail' == $column_name ) {
                // thumbnail of WP 2.9
                $thumbnail_id = get_post_meta( $post_id, '_thumbnail_id', true );
                // image from gallery
                $attachments = get_children( array('post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image') );
                if ($thumbnail_id)
                    $thumb = wp_get_attachment_image( $thumbnail_id, array($width, $height), true );
                elseif ($attachments) {
                    foreach ( $attachments as $attachment_id => $attachment ) {
                        $thumb = wp_get_attachment_image( $attachment_id, array($width, $height), true );
                    }
                }
                    if ( isset($thumb) && $thumb ) {
                        echo $thumb;
                    } else {
                        echo __('None');
                    }
            }
    }
    // for posts
    add_filter( 'manage_posts_columns', 'AddThumbColumn' );
    add_action( 'manage_posts_custom_column', 'AddThumbValue', 10, 2 );
    // for pages
    add_filter( 'manage_pages_columns', 'AddThumbColumn' );
    add_action( 'manage_pages_custom_column', 'AddThumbValue', 10, 2 );
}
?>