(a) To change the image, you have to replace the image file logo.png that is located in the directory \administrator\templates\bluestork\images.
(b) To change the link, edit the file index.php that is located in the directory \administrator\templates\bluestork and change the following line of code.
code:
<span class="logo"><a href="http://www.joomla.org" target="_blank"><img src="templates/<?php echo $this->template ?>/images/logo.png" alt="Joomla!" /></a></span>
but before this make a copy of you original code.
There are a lot of web tools that will help you with Joomla.