In the files (depending on your layout choice:

components > com_K2 > templates > user.php

components > com_K2 > templates > category_item.php

components > com_K2 > templates > latest_item.php

You need to change:

<?php echo JText::_('Be the first to comment!'); ?>

to 

<?php echo "<img src='$images/firstcomment.png' title='First Comment' alt='First Comment' />"; ?>

Change the same line in the ReadMore string to change that to an image too.