Theme admin_menu icon

You don't really need to, because it's set to use the favicon if you've provided one in your theme settings.

But to override this, as described in the admin_menu README, simply copy this function (hidden in admin_menu.inc at the time of this writing) to your template.php file:

<?php
/**
 * Render an icon to display in the Administration Menu.
 *
 * @ingroup themeable
 */
function theme_admin_menu_icon() {
  return '<img class="admin-menu-icon" src="'. (theme_get_setting('favicon') ? theme_get_setting('favicon') : url('misc/favicon.ico')) .'" width="16" height="16" alt="" />';
}
?>

And rename it to your theme or phptemplate_

Resolution