<?php

add_filter( 'manage_posts_columns', 'rt_add_gravatar_col');
function rt_add_gravatar_col($cols) {
	$cols['thumbnail'] = __('Thumbnail');
	return $cols;
}

add_action( 'manage_posts_custom_column', 'rt_get_author_gravatar');
function rt_get_author_gravatar($column_name ) {
	if ( $column_name  == 'thumbnail'  ) {
		echo get_the_post_thumbnail(get_the_ID(), array(100, 100));
	}
}

?>