Find these lines of code in item.php or .../templates/html/com_k2/templates/YourTemplateName/item.php

    <?php if($this->item->params->get('itemImageGallery') && !empty($this->item->gallery)): ?>
  <!-- Item image gallery -->
  <a name="itemImageGalleryAnchor" id="itemImageGalleryAnchor"></a>
  <div class="itemImageGallery">
	  <h3><?php echo JText::_('Image Gallery'); ?></h3>
	  <?php echo $this->item->gallery; ?>
  </div>
  <?php endif; ?>

Cut the code out and move it just above
	  <!-- Item introtext --> in the same file