Display post thumbnail for specific category outside the loop

Use get_the_post_thumbnail with a new WP_Query.

<?php 
 if ( has_post_thumbnail()) {
 $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large');
 echo '<a href="' . $large_image_url[0] . '" title="' . the_title_attribute('echo=0') . '" >';
 echo get_the_post_thumbnail($post->ID, 'thumbnail', array('class' => 'aligncentre')); 
 echo '</a>';
}
?>