Is there a singular location where all external notifications can be updated with a university logo?

We're trying to update our external notifications with our university logo in order to make them look more official (specifically our recruiting notifications, but all notifications will need to be updated eventually). Is there a singular location where all external notifications can be updated with a university logo? If so, how is that updated?

I tried updating the delivered rtf notification in Reports and Analytics, but the system threw an error saying delivered content cannot be updated. And we don't have the external notification in the custom folder.