Change display of featured image for pages in twenty seventeen theme

You are struggling with wrong files. You can find post files in
theme-folder/template-parts/post/content.php file.

You’ll find this code you can add your classes or whatever in your child theme.

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
    if ( is_sticky() && is_home() ) :
        echo twentyseventeen_get_svg( array( 'icon' => 'thumb-tack' ) );
    endif;
?>
<header class="entry-header">
    <?php
        if ( 'post' === get_post_type() ) :
            echo '<div class="entry-meta">';
                if ( is_single() ) :
                    twentyseventeen_posted_on();
                else :
                    echo twentyseventeen_time_link();
                    twentyseventeen_edit_link();
                endif;
            echo '</div><!-- .entry-meta -->';
        endif;

        if ( is_single() ) {
            the_title( '<h1 class="entry-title">', '</h1>' );
        } else {
            the_title( '<h2 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark">', '</a></h2>' );
        }
    ?>
</header><!-- .entry-header -->

<?php if ( '' !== get_the_post_thumbnail() && ! is_single() ) : ?>
   <!-- post thumbnail code -->
    <div class="post-thumbnail">
        <a href="<?php the_permalink(); ?>">
            <?php the_post_thumbnail( 'twentyseventeen-featured-image' ); ?>
        </a>
    </div><!-- .post-thumbnail -->
<?php endif; ?>

thumbnail section mentioned in html comments.