// genesis_entry_header action hook is used here because it is the one immediately above genesis_entry_content. Remember: Hook as late as possible.
add_action( 'genesis_entry_header', 'sk_custom_post_image' );
function sk_custom_post_image() {

	// if we are on portfolio CPT archive page and featured images are set to be shown on content archives in Genesis theme settings
	if ( is_post_type_archive( 'portfolio' ) && genesis_get_option( 'content_archive_thumbnail' ) ) {

		// remove the standard featured image output by Genesis per theme settings
		remove_action( 'genesis_entry_content', 'genesis_do_post_image', 8 );

		// add custom featured image output
		add_action( 'genesis_entry_content', 'sk_do_post_image', 8 );

	}

}

// Function to set the URL of featured image to value of a custom field, if present - otherwise to entry's permalink
function sk_do_post_image() {

		// store value of "item_url" custom field into a variable named "$item_url"
		$item_url = esc_url( get_post_meta( get_the_ID(), 'item_url', true ) );

		// if "item_url" custom field is empty, store the permalink in the "$item_url" variable
		if ( empty( $item_url ) ) {
			$item_url = get_permalink();
		}

		$img = genesis_get_image( array(
			'format'  => 'html',
			'size'    => genesis_get_option( 'image_size' ),
			'context' => 'archive',
			'attr'    => genesis_parse_attr( 'entry-image', array ( 'alt' => get_the_title() ) ),
		) );

		if ( ! empty( $img ) ) {
			printf( '<a href="%s" aria-hidden="true">%s</a>', $item_url, $img );
		}

}