Trying to edit archive.php to only show post extract, with featured image

To replace the full content with the excerpt, change

<?php the_content( '<em>Continue reading &rarr;</em>' ); ?>

To

<?php the_excerpt(); ?>

To display the featured image use this:

<?php the_post_thumbnail( 'single-post-thumbnail' ); ?>

Note: Make sure you have this next snippet somewhere in your theme’s functions.php file

//add theme support for post thumbnails
add_theme_support( 'post-thumbnails' );