First, you need to edit item.php of your template overrides. If you do not use already, create them by reading here:
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates
Open item.php and locate this code
<?php if($comment->userImage): ?>
<img src="<?php echo $comment->userImage; ?>" alt="<?php echo JFilterOutput::cleanText($comment->userName); ?>" width="<?php echo $this->item->params->get('commenterImgWidth'); ?>" />
<?php endif; ?>
replace it with this code
<?php
$findme = 'user.png';
$pos = strpos($comment->userImage, $findme);
if($comment->userImage && !$pos): ?>
<img src="<?php echo $comment->userImage; ?>" alt="<?php echo JFilterOutput::cleanText($comment->userName); ?>" width="<?php echo $this->item->params->get('commenterImgWidth'); ?>" />
<?php endif; ?>
