This article explains how to change a Google Docs background to a picture so you can add text over that picture.
How to Make an Image a Background on Google Docs
There are a couple of ways to add text on top of an image in a Google document. Here's how to insert an image in the background so that you can type over it in Google Docs:
How to Add Text to an Image With the Drawing Tool
Alternatively, you can add images to Google Docs with the Drawing tool and then add a text box over it.
Another option is to change the background in Google Slides, add your text to a slide, then take a screenshot. You can then insert the screenshot into Google Docs as an image.