The Checkout Page is standardized to maximize sales, and minimize errors in checkout. However, there are several text areas that can be customized within the checkout page to make for a more unique customer experience.
Customizable Checkout Content
Navigate to Account > Frontend
Checkout Summary Code
Once you are satisfied with what has been entered here, click "Save Frontend Prefs"
Uploading a Checkout Banner
Please refer to the article Upload a Checkout Banner