Windows Phone: Images dynamically added to PhoneApplicationFrame are not displayed - windows-phone-8

Question

I would use a portion of PhoneAplicationFrame for to display a set of controls on every page of my application.
One of these controls would be an Image control, which would display images obtained at runtime.
But the image never display its content.
By adding the same control in a grid of PhonePage it is displayed.
How can I do that?
APP.XAML:
<Application
x:Class="PhoneApp1.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone">
<Application.Resources>
<local:LocalizedStrings xmlns:local="clr-namespace:PhoneApp1" x:Key="LocalizedStrings"/>
<ControlTemplate x:Name="TheControlTemplate">
<Grid>
<ContentPresenter/>
<Grid x:Name="TheGridAbove"/>
</Grid>
</ControlTemplate>
</Application.Resources>
<Application.ApplicationLifetimeObjects>
<shell:PhoneApplicationService
Launching="Application_Launching" Closing="Application_Closing"
Activated="Application_Activated" Deactivated="Application_Deactivated"/>
</Application.ApplicationLifetimeObjects>
</Application>
APP.XAML.CS CONTRUCTOR
static public Grid TheGridAbove = null;
public App()
{
UnhandledException += Application_UnhandledException;
InitializeComponent();
InitializePhoneApplication();
InitializeLanguage();
if (Debugger.IsAttached){
Application.Current.Host.Settings.EnableFrameRateCounter = true;
PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;}
RootFrame.Template = Resources["TheControlTemplate"] as ControlTemplate;
RootFrame.ApplyTemplate();
TheGridAbove = (VisualTreeHelper.GetChild(RootFrame, 0) as FrameworkElement).FindName("TheGridAbove") as Grid;
}
MainPage.xaml
<phone:PhoneApplicationPage
x:Class="PhoneApp1.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneForegroundBrush}"
SupportedOrientations="Portrait" Orientation="Portrait"
shell:SystemTray.IsVisible="True">
<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="LayoutRoot" Background="Transparent">
<Button x:Name="button" Content="Button" HorizontalAlignment="Left" Margin="163,172,0,0" VerticalAlignment="Top" Click="button_Click"/>
</Grid>
</phone:PhoneApplicationPage>
MAINPAGE.XAML.CS button_Click
private void button_Click(object sender, RoutedEventArgs e)
{
Grid grid = new Grid();
grid.Background = new SolidColorBrush(Colors.Red);
Image img = new Image();
img.Source = new BitmapImage(new Uri("/Common/Lenna.jpg", UriKind.Relative));
grid.Children.Add(img);
PhoneApp1.App.TheGridAbove.Children.Add(grid); // DON'T DISPLAY
//LayoutRoot.Children.Add(grid); // THIS DISPLAY
}
CODE CAN BE DOWNLOADED HERE: http://www.filedropper.com/code