My goal is to attach a new image control while the application is running.
img = new System.Windows.Controls.Image();
img.Margin = new Thickness(200, 10, 0,
You should have only one root element under window. Adding the image using this.AddChilda adds the image as child of window, but you probably have some other child defined(Grid for example). Give a name to this child (Grid in the example) and then in the code behind add the image to the Grid
Example :
Then in the code behind use
RootGrid.Children.Add(img);