Customizing the Category Page

I'm not sure this is in the right sub-forum but there's not many choices!

I want to customize the main page area shown when the customer selects a category with sub-categories and no products. Default, it shows the sub categories as links in the main area. What files do I need to alter to affect this page? I want to add a main category photo, borders, and move the links around.

includes/templates/your template/templates/tpl_index_categories_default.php

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