If I've understood you correctly then you should open
templates/YOUR_TEMPLATE/common/tpl_main_page.php and locate
Code:
<?php
/**
* prepares and displays left column sideboxes
*
*/
?>
<div id="navColumnOneWrapper" style="width: <?php echo BOX_WIDTH_LEFT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_left.php')); ?></div></td>
and insert your image..
Code:
/**
* prepares and displays left column sideboxes
*
*/
?>
<div id="navColumnOneWrapper" style="width: <?php echo BOX_WIDTH_LEFT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_left.php')); ?></div><br /><br />HERE</td>
and do the same for the right column (you may also wish to put it inside a new <div> to sort our positioning, etc.)