WordPress: Show thumbnail in blog posts using Genesis Framework

functions.php

// insert into functions.php

// please activate featured image in Genesis settings

// show thumbnail in blog post
add_action( 'genesis_entry_header', 'single_post_featured_image', 15 );

function single_post_featured_image() {

    if ( ! is_singular( 'post' ) ) return;

    $img = genesis_get_image(
      array(
        'format' => 'html',
        'size' => genesis_get_option( 'image_size' ),
        'attr' => array( 'class' => 'post-image' )
      )
    );
    printf( '<a href="%s" title="%s">%s</a>', get_permalink(), the_title_attribute( 'echo=0' ), $img );
}

You'll find more code like this on the authors website https://wpsites.net/web-design/display-featured-image-before-or-after-entry-title-on-single-posts-pages/