First of all, to save yourself on future upgrades, the file changes should not be made to files in the template_default directory but instead the file should be copied to your template's directory and then modified from there.
As to adding the click portion to the image, you'll be adding an <a tag before the declaration/echo of the image and a closing </a> tag at an html validation point after the image. Here is the code around line 39 of the referenced file starting at line 28:
<!-- deprecated - to use uncomment this section
<?php if (TEXT_INFORMATION) { ?>
<div id="" class="content"><?php echo TEXT_INFORMATION; ?></div>
<?php } ?>-->
<?php if (DEFINE_MAIN_PAGE_STATUS >= 1 and DEFINE_MAIN_PAGE_STATUS <= 2) { ?>
<?php
/**
* get the Define Main Page Text
*/
?>
<div id="indexDefaultMainContent" class="content"><?php require($define_page); ?></div>
<?php } ?>
With this being line 39:
<div id="indexDefaultMainContent" class="content"><?php require($define_page); ?></div>
So if you wanted an image to appear before this content (so that it at the top) then you could add the following just before than line/moving that line down:
<a href="<?php echo zen_href_link('main_page_name', 'parameters', $request_type); ?>" ><?php echo zen_image('path_to_image', 'alt_text', 'image_width_if_desired', 'image_height_if_desired', 'parameters_if_desired'); ?>
This would provide a little dynamic capability to the site such that if the code and files of this site are moved elsewhere, then the links and images should still function normally.
