functions.php

// Register a custom image size for Singular Featured images
add_image_size( 'singular-featured-thumb', 800, 250, true );

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