template-portfolio.php

<?php
/* Template Name: Portfolio */

/**
 * Portfolio Listing
 *
 */
function be_portfolio_listing() {
	$args = array(
		'post_type' => 'portfolio',
		'posts_per_page' => -1,
	);
	$loop = new WP_Query( $args );
	if( $loop->have_posts() ): while( $loop->have_posts() ): $loop->the_post();
		$class = 0 == $loop->current_post || 0 == $loop->current_post % 2 ? 'one-half first' : 'one-half';
		echo '<div class="' . $class . '"><a href="' . get_permalink() . '">' . get_the_post_thumbnail( get_the_ID(), 'thumbnail' ) . '</a></div>';
	endwhile; endif; wp_reset_postdata();
}
add_action( 'genesis_after_loop', 'be_portfolio_listing' );

genesis();