Why are Letterhead images not appearing in my sandbox testing?
The Merchant Facility related to the Payment Txn that you are testing needs to be a valid Merchant Facility. This means, it needs to be Active and belong to have the Salesforce Org Id be the same as the Sandbox Salesforce Org Id.
PLEASE Read some very important Instructions BEFORE Updating the Salesforce Org Id on Merchant Facilities. Please see procedure I created a new Full or Partial Sandbox, what do I need to do to make things work? - In particular the note about removing all Merchant Facility payment gateway passwords.