When you create a "template image" in a docx for XDocReport, you must insert a bookmark inside the image. For instance if your image is inserted in a table cell, your bookmark must be inserted inside the image and not inside the table cell, otherwise the generated docx will not be valid.
It think it's your problem, see at https://code.google.com/p/xdocreport/issues/detail?id=340#c7 for more information.