You can not use your templates, as the default Odoo reports are written in QWeb. But you can create company report headers and footers. 

This web helps me with the issue.  Has two examples, text and image:

https://www.excelroot.com/post/how-to-add-watermark-in-the-odoo-report-pdf

Cheers

Hi,

Please check this free module: Reports with Watermark

Thanks

Hi Ricardo,

You can achieve that by customizing your reports by following code.

<div class="watermark_logo" style="position:absolute;opacity:0.30;front-size:8em;width:100%;text-align:center;z-index:100;">
    <t t-if='doc.company_id.logo'>
        <img t-if="doc.company_id.logo" t-att-src="image_data_uri(doc.company_id.logo)" alt="Logo"/>
    </t>
</div>

Regards,

Keyur Makwana.