single-post.php

<?php

// Add "has-featured-image" body class if the post has a featured image
add_filter( 'body_class', 'sk_single_post_body_class' );
/**
 * Adds a css class to the body element
 *
 * @param  array $classes the current body classes
 * @return array $classes modified classes
 */
function sk_single_post_body_class( $classes ) {

	if ( has_post_thumbnail() ) {
		$classes[] = 'has-featured-image';
	}

	return $classes;
}

add_action( 'get_header', 'sk_layout' );
function sk_layout() {

	// if the post does not have a featured image, abort.
	if ( ! has_post_thumbnail() ) {
		return;
	}

	// force full width content
	add_filter( 'genesis_pre_get_option_site_layout', '__genesis_return_full_width_content' );

	// insert featured image and Primary sidebar
	add_filter( 'the_content', sk_featured_image_sidebar, 20 );

}

// Function to insert featured image and Primary sidebar
function sk_featured_image_sidebar( $content ) {

	// store the Primary sidebar's output in a variable using output buffering
	ob_start();
		get_sidebar(); // include the sidebar.php template file
	$sidebar = ob_get_clean();

	if ( wp_is_mobile() ) {
		// insert featured image after the first paragraph
		$content = preg_replace( '/<\/p>/', '</p>' . get_the_post_thumbnail( $post->ID, 'post-single' ), $content, 1 );
	} else {
		// insert featured image and Primary sidebar after the first paragraph
		$content = preg_replace( '/<\/p>/', '</p>' . get_the_post_thumbnail( $post->ID, 'post-single' ) . $sidebar, $content, 1 );
	}

	if ( wp_is_mobile() ) {
		return $content . $sidebar;
	} else {
		return $content;
	}

}

genesis();