Genesis show featured image in single post

functions.php

add_action( 'genesis_entry_content', 'single_post_featured_image', 1 );
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' ) ) );
    
    echo $img;
}