just before or after the content of the post or main page
in the indes.php
<p align="center"><?php the_post_thumbnail(); ?></p>
<?php the_content('Read the rest of this entry »'); ?>
in the single.php
<p align="center"><?php the_post_thumbnail('single-post-thumbnail'); ?></p>
<?php the_content('<p class="serif">Read the rest of this entry »</p>'); ?>