A recent request came in asking for a sample demonstrating how to set the XamDataTree's ExpandedIconTemplate in code.  The attached sample here demonstrates how you can create a FrameworkElementFactory object using an image and setting this as the DataTemplate's VisualTree property.

Hopefully, this can serve as a guide for other programmers wishing to do the same...

        DataTemplate dt;

        public MainWindow()
        {
            InitializeComponent();

            dt = new DataTemplate();
            BitmapImage bi = new BitmapImage(new Uri(@"../../images/Sunset05.JPG", UriKind.Relative));
            FrameworkElementFactory imageElementFactory = new FrameworkElementFactory(typeof(Image));
            imageElementFactory.SetValue(Image.SourceProperty, bi);
            dt.VisualTree = imageElementFactory;
            dt.Seal();
            this.xamDataTree1.ExpandedIconTemplate = dt;
        }

The result when running hte sample is that the image I specified is used for the expanded nodes' icon as below.