bushidO, попробуй вот так:
echo '<a href="index.php?cat='.$cat.'&orderby='.($order == 'desc' ? 'asc' : 'desc').'" title = "'.($order == 'desc' ? dow['descending'] : $dow['ascending']).'"><img src="img/'.($order == 'desc' ? 'asc' : 'desc').'.png" alt="'.($order == 'desc' ? $dow['descending'] : $dow['ascending']).'" /></a>';