WQP

function nthChild($multiplier, $addition, $max)
{
    $validInedexes = array();

    for ($n = 0; $n < $max; $n++)
    {
        $idx = $multiplier * $n + $addition;
        if ($idx > $max)
        {
            return $validInedexes;
        }
        $validInedexes[] = ($idx - 1);
    }
}

.menu img{
  display: block;
  max-width: 100%;
  margin: 0px auto;
}