Enum Alignment

// Open a form by ID and add a new image item with alignment
const form = FormApp.openById('1234567890abcdefghijklmnopqrstuvwxyz');
const img = UrlFetchApp.fetch('https://www.google.com/images/srpr/logo4w.png');
form.addImageItem().setImage(img).setAlignment(FormApp.Alignment.CENTER);