add_action( 'wp_head', 'sk_add_archives_image' );
function sk_add_archives_image() {
	if ( class_exists( 'MultiPostThumbnails' ) ) {
		new MultiPostThumbnails(
			array(
				'label' => 'Archives Image',
				'id' => 'archives-image',
				'post_type' => 'post'
			)
		);
	}
}

add_action( 'genesis_before_entry', 'sk_archives_image' );
function sk_archives_image() {

	// if we are on a single page, abort.
	if ( is_singular() ) {
		return;
	}

	if ( class_exists( 'MultiPostThumbnails' ) && MultiPostThumbnails::has_post_thumbnail( 'post', 'archives-image' ) ) {
		remove_action( 'genesis_entry_content', 'genesis_do_post_image', 8 );

		add_action( 'genesis_entry_content', function () {
			MultiPostThumbnails::the_post_thumbnail( 'post', 'archives-image', null, 'medium', array( 'class' => 'alignleft' ) );
		}, 8 );
	}

}