Allow each blog post to have different featured images

I am want to be able to allow each new post to have a different feature image.

Right now when I change it on one post it changes on all the posts.

This is the code I am currently running

<?php if ( has_post_thumbnail()) : ?>
   <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">
     <?php the_post_thumbnail(); ?>
   </a>
<?php endif; ?>