Well, I'm very new to Zen Cart but I did it in a different way and it seems to work fine.
My site is in English and Greek. I installed the Greek language pack and now have two header.php files:
/includes/languages/english/my_template/header.php
/includes/languages/greek/my_template/header.php
I created two banners, placed them in /includes/templates/my_template/images and named them logo_english.gif and logo_greek.gif.
In the /includes/languages/english/my_template/header.php file I changed the line
define('HEADER_LOGO_IMAGE', 'logo.gif'); to
define('HEADER_LOGO_IMAGE', 'logo_english.gif');
and in the /includes/languages/greek/my_template/header.php file I changed the line
define('HEADER_LOGO_IMAGE', 'logo.gif'); to
define('HEADER_LOGO_IMAGE', 'logo_greek.gif');
That was all. You can see it in action at http://athenscollectibles.info (the text on the right side of the logo changes with the language).
As I said at the beginning, I'm very new to Zen Cart and I'm still not confident. If one of the many experts reads this, I would appreciate an advice if this was correctly done or whether I should expect something nasty to happen.