Which is the theme template file that contains the logo image of the site?
The $logo variable is printed from page.tpl.php around line 77.

&lt;?php if ($logo): ?&gt;
  &lt;a href="&lt;?php print $front_page; ?&gt;" title="&lt;?php print t('Home'); ?&gt;" rel="home" id="logo"&gt;&lt;img src="&lt;?php print $logo; ?&gt;" alt="&lt;?php print t('Home'); ?&gt;" /&gt;&lt;/a&gt;
&lt;?php endif; ?&gt;


Depending on what you want to do though you could also use template_preprocess_page().