Enum Alignment

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