The original JA Purity template was provided from JoomlArt.com as a zip but the latest files have been installed by default 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 change the size of the logo you'll need to change the size of the header area and edit the relevant style sheet, template.css which is in the following directory:
templates/ja_purity css/ template.css
If you've been following the tutorial version then the directory is:
templates/my_japurity css/ template.css
You can edit the CSS file directly in your choice of plain text editors such as VI or Notepad or you can use the CSS Editor provided in the Joomla Administration area.