너무 감사합니다~ 

<?php foreach($content->attach as $key => $attach):?>
<p class="thumbnail-area"><img src="<?php echo get_site_url() . $attach[0]?>" alt="<?php echo $attach[1]?>"></p>
<?php endforeach?>

아래 코드로 바꿔 보시겠어요?

<?php $index=0; foreach($content->attach as $key => $attach): if($index>0):?>
<p class="thumbnail-area"><img src="<?php echo get_site_url() . $attach[0]?>" alt="<?php echo $attach[1]?>"></p>
<?php endif; $index++; endforeach?>

$index 값을 줘서 첫 번째는 출력하지 않도록 했습니다 :)