register_custom_layout_genesis.php

<?php // <- Remove <?php if your using this snippet

// Register New Layout
function jm_custom_layout() {
    genesis_register_layout( 'layout-name', array(
        'label' => __('custom/layout/name', 'genesis'),
        'img' => get_bloginfo('stylesheet_directory') . '/images/yourlayout.gif'
    ) );
}
add_action( 'init', 'jm_custom_layout' );