Hi Isio,

You can simply create a new div and insert an image in it. Make sure its the width & height of the page and give it a z-index. It'll work. Something along these lines:

 <div class="backgroundimage" style="position:absolute;opacity:0.25;font-size:3em;width:100%;height:100%;z-index:1000;">
        <img src="yourpath/yourfile.jpg"/>
        <!-- Or an image from the db. For example: --->
        <p t-field="o.client_order_ref"/>
 </div>

I hope this gives you enough information and a general idea about how to do it. Its simply a matter of HTML and css!
Note: this is untested code, its just to give you an idea.

Yenthe