The thread "Can I make background Image clickable link?" was closed. I got hint from eric47905's post and applied on my site. If you think this could be helpful, just take it.
Here http://angelcorp.net/infant-formula/...products_id=88 The background image is not clickable.
And here http://angelcorp.net/test/zencart/in...products_id=92 It's clickable, and the background image and logo point to different url.
What I did is in "/includes/templates/your_template/common/tpl_header.php" I made it

<!--bof-branding display-->
<div id="logoWrapper">
<div id="logo"><?php echo '<a href="http://angelcorp.net/trade/" target="_blank">' . zen_image($template->get_template_dir(HEADER_LOGO_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . HEADER_LOGO_IMAGE, HEADER_ALT_TEXT) . '</a>'; ?></div>

<a id="bgWrapper" title="Angel Store / 安吉网店" href="http://angelcorp.net/test/zencart/"> &nbsp; </a>

<?php if (HEADER_SALES_TEXT != '' || (SHOW_BANNERS_GROUP_SET2 != '' && $banner = zen_banner_exists('dynamic', SHOW_BANNERS_GROUP_SET2))) { ?>
<div id="taglineWrapper">

And in "/includes/templates/your_template/css/stylesheet.css" I made it

#logoWrapper {
background-image: url(../images/GrandCanyon.jpg);
height: 150px;
margin: 11px 0px -1px 0px;
}

#bgWrapper {
background-image: url(../images/GrandCanyon.jpg);
height: 150px;
margin: 11px 0px -1px 0px;
display: block;
}

I got a lot of hints and help from the forum. Thanks to those contributors. Enjoy your site construction.