In the reports, Letter Head is fetched from the Company master. To have company's Letter Head fetched correctly in the report, please ensure that you have updated default Letter Head in the Company master.
If you are managing multiple companies in a single ERPNext account, then ensure that for each Company, default Letter Head is set in the Company master.
After updating Letter Head in the Company master, refresh your ERPNext account, and then check the print format of a report.