You can add a custom banner image to match your brand on your checkout page. To add a customer banner click the "Edit" button next to the product you want to add the banner to.
Next select the "Checkout" tab.
You have to select either the “Clean Simple” or “Modern Orange” checkout template.
Once selected the ability to add images will populate on the page as shown here: 
Once you have added your banner, scroll to the bottom of the page and click the "Save as Draft and Go to the Next Step" button. You will then need to click on the Review tab if you have made all your changes and either click "Save as Draft" if you are not done with your product or "Submit" if your product is done and ready to be reviewed by Compliance.