have a look in includes/languages/english/your_template/header.php
at line 21 you should have this:
// added defines for header alt and text
define('HEADER_ALT_TEXT', 'Home');
define('HEADER_SALES_TEXT', '');
define('HEADER_LOGO_HEIGHT', '77px');
define('HEADER_LOGO_WIDTH', '509px');
define('HEADER_LOGO_IMAGE', 'logo.gif');
just set your alt text in there (change Home to whatever you want your alt text to say)
if you dont have a file in includes/languages/english/your_template/ called header, just copy it from includes/languages/english/
and put it in includes/languages/english/your_template/
(your_template being the name of the template you are using)
if that doesnt add the alt text, then check this file
includes/templates/your-template/common/tpl_header.php
and look for this code at around line 54
<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, HEADER_LOGO_WIDTH, HEADER_LOGO_HEIGHT) . '[/url]'; ?></div>
notice the HEADER_ALT_TEXT
that is the call to the alt text written into the header.php in your language files, if that is missing from your tpl_header.php in your template files, it wont pull the alt text on to your header image.
it should just be a matter of checking includes/languages/english/your_template/header.php
or adding it to your template, but if that doesnt work, check the tpl_header.php in
includes/templates/your_template/common/
