Custom Columns WordPress Admin

first create the new column:

add_filter( 'manage_edit-post_columns', 'c3m_new_column');
function c3m_new_column( $defaults ) {
    $dfaults['post_thumbnail'] = 'Post Thumbnail';
    return $defaults;
}

Now the function to output the thumbnails in the column:

add_action( 'manage_posts_custom_column', 'c3m_custom_column', 10, 2);
function c3m_custom_column( $column_name, $post_id ) {
    if ($column_name == 'post_thumbnail') {
        if ( has_post_thumbnail() ) {
            $img_url = wp_get_attachment_image_src( get_post_thumbnail_id() );
            echo '<img src="'. esc_url( $img_url[0] ).'" />';
        } else { echo 'No Post Thumbnail Set'; }
    }