While there are many ways to do this the following is one:
Edit a copy of your tpl_header.php file for your template and add the following div
Code:
<!--bof-header logo and navigation display-->
<?php
if (!isset($flag_disable_header) || !$flag_disable_header) {
?>
<div id="headerWrapper">
<!--bof-branding display-->
<div id="logoWrapper">
<div id="logo"><?php echo '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '">' . zen_image($template->get_template_dir(HEADER_LOGO_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . HEADER_LOGO_IMAGE, HEADER_ALT_TEXT) . '</a>'; ?></div>
<div id="logo2"><img src="url(../images/your_image.xxx" width="??" height="??")"></div>
<div id="taglineWrapper">
<?php
if (HEADER_SALES_TEXT != '') {
?>
<div id="tagline"><?php echo HEADER_SALES_TEXT;?></div>
and to your stylesheet.css for your template add
Code:
#logo2 {
float: right;
}
This is the minium and if alignment is not correct the easiest is to match image heights of your logo.gif and the additional image in your graphics program.