Still using Odoo 8 I'm facing a problem with the background image in Odoo 13.

In 8 I did it like that in views/external_layout_header:

<?xml version="1.0"?>
    <t t-name="report.external_layout_header">
        <div class="header">
            <img src="/web/static/src/img/my_fullsize_background_image.png" name="Briefpapier"             width="110%" style="position: absolute; top: 0cm; left: -1cm" border="0" />
    </div>
</t>

How to get the same in Odoo 13?
Tried now for hours without any success. The image is either not show, only shown in one section (div), overlaping to the next page ...
in views/external_layout_xxx as well as in single reports like report_saleorder_document

Any helpful feedback is welcome.

BR