RectangleGeometry xRect = new RectangleGeometry();
            xRect.Rect = new Rect(100, 100, 100, 100);
            xRect.RadiusX = 20;
            xRect.RadiusY = 20;

            Image xImage = new Image();
            xImage.Source = new BitmapImage(new Uri("1.jpg", UriKind.Relative));
            xImage.Stretch = Stretch.Fill;
            xImage.Width = 300;
            xImage.Height = 200;

            xImage.Clip = xRect;

            LayoutRoot.Children.Add(xImage);

This one works for images