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'; }
}