Hi there.
Edit the file "/templates/your_template/html/com_k2/templates/custom_template/item.php"
You should use if and else. Change the code you are using for "item image XL Large" for this:
<!-- Check if there's a video and show it -->
<?php if($params->get('itemVideo') && !empty($this->item->video)): ?>
<div class="itemVideoBlock" id="itemVideoAnchor">
<h3><?php echo JText::_('K2_MEDIA'); ?></h3>
<?php if($this->item->videoType=='embedded'): ?>
<div class="itemVideoEmbedded"> <?php echo $this->item->video; ?> </div>
<?php else: ?>
<span class="itemVideo"><?php echo $this->item->video; ?></span>
<?php endif; ?>
</div>
<!-- If there's no video check if there's image and show it in XL Large size -->
<?php else: ?>
<?php if($params->get('itemImage') && !empty($this->item->image)): ?>
<div class="itemImageBlock">
<img src="<?php echo $this->item->imageXLarge; ?>" alt="<?php if(!empty($this->item->image_caption)) echo K2HelperUtilities::cleanHtml($this->item->image_caption); else echo K2HelperUtilities::cleanHtml($this->item->title); ?>" style="width:<?php echo $this->item->imageWidth; ?>px; height:auto;" />
</div>
<?php endif; ?>
<?php endif; ?>
That should do the trick.
Best luck!
