This is the content of includes/languages/english/html_includes/classic/define_main_page.php. Please note the line I've highlighted in red. This is where the image and link is coming from. If you create your own custom template, this line should disappear.
Code:
<a href="http://www.lulu.com/content/466605"><img src="images/large/e-start-book.gif" alt="get your manual today" title="Have you got yours yet? Join the 1000’s of Zen Cart users that have bought the only comprehensive owners manual !" /></a>
<p>This content is located in the file at: <code> /languages/english/html_includes/classic/define_main_page.php</code></p>
<p>You can quickly edit this content via Admin->Tools->Define Pages Editor, and select define_main_page from the pulldown.</p>
<p><strong>NOTE: Always backup the files in<code> /languages/english/html_includes/your_template</code></strong></p>
You can use admin>tools>developers toolkit to find items like this.
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