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 files, logo.gif (used for PNG transparency problems) and logo.png, are in the following directory:
templates/ja_purity images/ logo.gif logo.png
If you've been following the tutorial you can download a tutorial version of the template that installs to the following directory:
templates/my_japurity images/ logo.gif logo.png
If you decide later to change the size of the header area then you'll want to change the size of your images to match.