Customize Single Post Featured Image and Add Before Entry

functions.php

<?php
// Do NOT include the PHP opening tag

// Register a custom image size for Singular Featured images
add_image_size( 'singular-featured-thumb', 800, 450, true );

// Display featured image before entry
add_action( 'genesis_before_entry', 'display_featured_image' );
function display_featured_image() {
	if ( ! is_singular( array( 'post' ) ) ) {
		return;
	}
	if ( ! has_post_thumbnail() ) {
		return;
	}
	
	// Display featured image above content
	echo '<div class="singular-featured-image color-gradient">';
		genesis_image( array( 'size' => 'singular-featured-thumb' ) );
	echo '</div>';
}