index.html is the main file that is be used to load a template. You can include as many assets and helper files (Javascript, CSS, images) as required in your templates. You can create subfolders to manage your files accordingly.
Template Assets
To refer to your assets, use the merge tag: {{template.baseUrl}}. For example to access the background.png image (from the screenshot), you would use the following code:
<img src="{{template.baseUrl}}/images/background.png" alt="" class="image-bg">
Global Assets
You can make use of the global folder in the venue to store assets that can be shared across all templates. To refer to the assets under the global folder, use the merge tag: {{global.baseUrl}}.