Just set the CSS top and left properties on the image, along with the appropriate positioning value to position the image over the text. Opacity might also help if you want them to be able to see the text through the image.
You can set the CSS in the story's Stylesheet or in the "style" parameter on an "img" element.
If you're having trouble getting the positioning correct, right-click on the image and click "Inspect". Then you can play around in the "Styles" section until you find the right values to put in your code so it appears where you want it to appear.