The section how to automatically extract thumbnail？？

related question here:
https://wordpress.stackexchange.com/questions/45529/retrieve-1st-image-in-post-and-set-it-as-featured-image-when-post-is-saved-upda

maybe it contains some code you can use (?)

The following piece of code how to modify in order to automatically extract thumbnail?

<?php $homethumb = get_post_meta($post->ID,'homethumb', true); ?>
<img rel="nofollow" src="https://www.website.com/img/files/<? echo strtolower($homethumb); ?>.jpg" width="" height="" alt=""  />
<center><b><a>" title="<?php the_title(); ?>"><?php the_title(); ?></a></b></center>

<center><?php the_excerpt(); ?></center>