Hi,

Modifying the Invoice PDF report in Odoo V19 online to add a full-page background image is challenging due to the limitations of the SaaS platform, which restricts custom code. While direct modification is not possible, you can explore workarounds using Odoo Studio, such as adding a watermark image that covers most of the page. If you're sending invoices through the Odoo Website, you might be able to create a custom template using the Website Builder.

For more flexibility, consider migrating to Odoo.sh or an on-premise installation, which allows you to develop a custom module with a new QWeb view to add the background image using CSS. Alternatively, check the Odoo App Store for third-party apps that offer advanced PDF customization. Evaluate the cost and effort of these options against the value of having the full-page background image.

Reference:-

* https://www.youtube.com/watch?v=5jrwei9tM8s

* https://www.youtube.com/watch?v=T6DjVXwFxPM

* https://www.cybrosys.com/blog/how-to-customize-views-reports-in-odoo-18-studio

* https://www.cybrosys.com/blog/how-to-customize-views-and-reports-in-odoo-16-studio-module

Hope it helps