Adding a banner to the top of your calendar or a custom graphic on the page is not currently available as a feature in PrintMyCal. (Perhaps it will be in the future!)
You can still customize your calendar with a banner, graphic, or other changes, but it will need to be done in your word processor. Just download the RTF version of your calendar from “Step 4” of PrintMyCal and open the file in your word processor. From there you should be able to make any changes that you would like. Look for a helpful guide on the internet if you need instructions for your particular word processor.