functions.php

add_action( 'genesis_before_loop', 'sk_portfolio_archive' );
function sk_portfolio_archive() {
	if ( ! is_post_type_archive( 'portfolio' ) ) {
		return;
	}
	remove_action( 'genesis_loop', 'genesis_portfolio_setup_loop', 9 );
	add_action( 'genesis_loop', 'sk_portfolio_setup_loop', 9 );
}

function sk_portfolio_setup_loop() {

	$hooks = array(
		'genesis_before_entry',
		'genesis_entry_header',
		'genesis_before_entry_content',
		'genesis_entry_content',
		'genesis_after_entry_content',
		'genesis_entry_footer',
		'genesis_after_entry',
	);

	foreach ( $hooks as $hook ) {
		remove_all_actions( $hook );
	}

	add_action( 'genesis_entry_content', 'sk_portfolio_grid' );

}

/**
 * Callback on the `genesis_portfolio_grid` action.
 * Verifies there is an image attached to the portfolio item
 * then outputs the HTML for the image with classes for styling.
 *
 * @uses genesis_get_image()
 *
 * @access public
 * @return void
 */
function sk_portfolio_grid() {

	$image = genesis_get_image( array(
			'format'  => 'html',
			'size'    => 'portfolio',
			'context' => 'archive',
			'attr'    => array ( 'class' => 'portfolio-image' ),
		) );

	if ( $image ) {
		$title = apply_filters( 'genesis_post_title_text', get_the_title() );

		printf( '<div class="portfolio-item"><a href="%s" rel="bookmark">%s<h2 class="entry-title" itemprop="headline">%s</h2></a></div>', get_permalink(), $image, $title );
	} else {
		echo '<div class="portfolio-item">';
			genesis_do_post_title();
		echo '</div>';
	}

}