The best and easiest way to add a background image to any Google Docs is by using the Watermark feature.
You'll find this feature, which was added in late 2021, in the Google Docs menu under Insert > Watermark.
The Watermark sidebar will appear with two tabs. On the Image tab click on Select Image to choose an image.
You can choose an image from your Google Drive, Photos, Google Images, By URL, Camera, or Upload. Please keep in mind that you always want to use images to which you have the copyright.
Once you have added your image you can change the formatting to a certain degree. For instance, you can choose to have the background imaged displayed faded.
Once you are happy with the settings click on Done to close the Watermark sidebar.
If you want us to walk you through these steps watch our short video tutorial.