스킨의 document.php 파일을 수정해주세요 :)

<?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?>

위 코드를 아래로 교체 해보시겠어요?

<?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?>