Give this a try
double actualHeight = Application.Current.Host.Content.ActualHeight;
double actualWidth = Application.Current.Host.Content.ActualWidth;
Grid grid = new Grid();
grid.VerticalAlignment = VerticalAlignment.Center;
grid.HorizontalAlignment = HorizontalAlignment.Center;
grid.Height = actualHeight; //set height
grid.Width = actualWidth; //set width
grid.Background = new SolidColorBrush(Colors.White);
Image img = new Image();
img.VerticalAlignment = VerticalAlignment.Center;
img.HorizontalAlignment = HorizontalAlignment.Center;
img.Source = bitmapImage;
img.Tap += OnFullScreenImageTap;
RotateTransform rt = new RotateTransform
rt.CenterX = actualWidth / 2;
rt.CenterY = actualHeight / 2;
if (imageWidth > imageHeight)
{
//Landscape
rt.Angle = 0;
Debug.WriteLine("Display image in LANDSCAPE");
}
else if (imageWidth == imageHeight)
{
//Portrait
rt.Angle = 90;
Debug.WriteLine("Display image in PORTRAIT");
}
else
{
//Portrait
rt.Angle = 90;
Debug.WriteLine("Display image in PORTRAIT");
}
grid.Children.Add(img);
popUp.Child = grid; //set child content
this.LayoutRoot.Children.Add(popUp);
popUp.IsOpen = true;