We will use "Include PHP Code" module to manage this. . Your template will be recognized automatically by system. But it has to know, this file is a template. Your template need a name.
What this code mean? For homepage, with ID 1_1, system will use image "homepage_header.jpg", for page with ID 1_2, it'll use "subpage_header.jpg" and for any additional subpage, it'll use default image, "header.jpg"
Now, edit template file. You can use built-in template editor, but we recommend HTML editor, where you have more control and you can undo your changes. Find an image's definition in your template, and if it's <img src="header.jpg" />, replace it with this code: <img src="{%include_header%}" />, then save changes and upload your template to server. Changes should be viewable in the same moment.
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