The original JA Purity template was provided from JoomlArt.com as a zip but the latest files have been installed along with Joomla.
The relevant file is in the following directory:
If you've been following the tutorial you can download a tutorial version of the template that installs to the following directory:
Change this line:
<div id="ja-header" class="clearfix" style="background: url(<?php echo $tmpTools->templateurl(); ?>/images/header/<?php echo $tmpTools->getRandomImage(dirname(__FILE__).DS.'images/header'); ?>) no-repeat top <?php if($this->direction == 'rtl') echo 'left'; else echo 'right';?>;">
<div id="ja-header" class="clearfix">