Website Search Results Not Showing Images

Look here:

if($row->type=='post'){
    $result = wp_get_attachment_image_src ($attachment_id, 'medium');
    $url_img = $result['url'];

    $permalink = get_permalink($row->id);
}

You are using attachment_id variable, but you have not initiated it anywhere.

if($row->type=='post'){
    $attachment_id = get_post_thumbnail_id($row->id);
    $result = wp_get_attachment_image_src ($attachment_id, 'medium');
    $url_img = $result['url'];

    $permalink = get_permalink($row->id);
}