It's in the shipping module, in include/modules/shipping/austpost.php. Good luck, it is unlikely you will be able to fix it from there, unless you remove the images altogether and have a decent understanding of PHP; it is a very complex module. I just posted a simple question in the same ballpark. (The order total generator file is tpl_module_order_totals.php.) You can remove the entire shipping title and replace it with a generic one perhaps.
Well, just removing that one header image, on second reading, that is much more possible - still a decent basic of PHP would be necessary.
Good luck.
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