foreach ( $images as $attach_id ) {
    $i++;
    if ($attach_id > 0) {
        $post_thumbnail = wpb_getImageBySize(array( 'attach_id' => $attach_id, 'thumb_size' => $img_size ));
        $post_thumbnail_link = wp_get_attachment_image_src( $attach_id, 'full');
    }
    else {
        $different_kitten = 400 + $i;
        $post_thumbnail = array();
        $post_thumbnail['thumbnail'] = '<img src="http://placekitten.com/g/'.$different_kitten.'/300" />';
        $post_thumbnail['p_img_large'][0] = 'http://placekitten.com/g/1024/768';

        $post_thumbnail_link = array();
        $post_thumbnail_link[0] = null;
    }

    $thumbnail = $post_thumbnail['thumbnail'];
    $p_img_large = $post_thumbnail['p_img_large'];
    $link_start = $link_end = '';

    if ( $onclick == 'link_image' ) {
        $link_start = '<a href="'.$post_thumbnail_link[0].'"'.$pretty_rel_random.'>';
        $link_end = '</a>';
    }