Adding ALT tag to image in header

[bhensarl]

I want to add an ALT tag to an image in my header but I can't figure out where to add it. Here's the code in the tpl_header.php file.

<a href="<?php echo zen_href_link(FILENAME_DEFAULT);?>"><?php echo zen_image(DIR_WS_TEMPLATE.'images/logo.png'); ?></a><br />

Which file do I add the info to? Thanks.
Brian