Create your own properly sized images and upload them to your /images folder
Then in your template's copy of /languages/english/your_template/header.php find the text for them such as:
Code:
define('HEADER_TITLE_CATALOG', 'Home');
Replace 'Home' with imageto resemble
Code:
define('HEADER_TITLE_CATALOG', '<img src="images/your_image.XXX" alt="Your_Alt_Text" />');
Same for logon & others
If you need to find the file where the defines for the text is use the developers tools
https://www.zen-cart.com/tutorials/index.php?article=38
https://www.zen-cart.com/tutorials/index.php?article=39