To get new hosting accounts to show your custom designed index page, upload your index page with associated graphic images and style sheet where applicable to the cpanel3-skel folder under your account using an FTP client.
The cpanel3-skeldirectory is what will be used as a skeleton for new accounts. For example if you place an index.html file in /your-account-name/cpanel3-skel/public_html, and then setup a new account, that account will have a copy of your index.html in their public_html directory.