The code located in tpl_header.php can only affect the header area. To apply it to the whole page, you will need to move the code.
Does the top part of the gif look like the regular header? If so, it could be switching and the change not be visible, because it is cut off below the header.
You will want to edit /includes/templates/your_template/common/tpl_main_page.php. Near the top, find
PHP Code:
}
?>
<div id="mainWrapper">
<?php
/ **
* prepares and displays header output
and change
<div id="mainWrapper">
to this:
PHP Code:
<!--Smart Backgrounds-->
<?php $smart_image = '';
if ($current_page_base == 'index' or $current_page_base == 'product_info') { //add _ and top cat id to bg filename only if cat bg image exists
$smart_image = (file_exists(DIR_WS_TEMPLATE_IMAGES . 'smartbg_' . str_replace(strstr($_GET[cPath],'_'),'',$_GET[cPath]) . '.gif'))?'_' . str_replace(strstr($_GET[cPath],'_'),'',$_GET[cPath]):'';
} else { //add _ and page base to bg filename only if page bg image exists
$smart_image = (file_exists(DIR_WS_TEMPLATE_IMAGES . 'smartbg_' . $current_page_base . '.gif'))?'_' . $current_page_base:'';
}?>
<!--/Smart Backgrounds-->
<div id="mainWrapper" class="smartBG<?php echo $smart_image;?>">
and change the stylesheet to match, with #mainWrapper instead of #headerWrapper.