I extended the \'report.external_layout_footer\' qweb view to display image.
Below is my code in the file reports/external_layout.xml:
In case you want to use an image that isn't static, here's what you can do instead.
Using the company logo as an example:
Using the mime type "image/*" will let you use different format of image and not just jpeg or just png.
Then as odoo render by default binary data as base64, you can simply append the content of the image after base64,.