How set background image for doc files when exporting nodes to Word in Drupal 7?
The functionality you are looking for does not (as right now) exist in the version of the module you have linked to.

Therefore you will not be able to set the background image of the Word document without making modifications to the module. 

This would be non-trivial as your modified module would have to do the following:


parse the CSS of the page to determine what image should be displayed
load the image and embed it in a format that Microsoft Word can understand
set the necessary tags in the Word document to use the embed image