function signReports() {
  var folderId = '<<PUT THE ID OF YOUR FOLDER WITH DOCS TO APPEND IMAGES TO>>';
  var imageId = '<<PUT THE ID OF THE IMAGE TO APPEND HERE>>';

  var files = DriveApp.getFolderById(folderId).getFilesByType(MimeType.GOOGLE_DOCS);

  while(files.hasNext()) {
    var file = files.next();
    var doc = DocumentApp.openById(file.getId());
    var image = DriveApp.getFileById(imageId);

    doc.getBody().appendImage(image).setWidth(doc.getBody().getPageWidth());
  }
}