You are all wrong! Why? Because all you need is this code to work:
(image View) / C# Img is : your Image box
Keep this as is, without change ("ms-appx:///) this is code not your app name Images is your folder in your project you can change it. dog.png is your file in your folder, as well as i do my folder 'Images' and file 'dog.png' So the uri is :"ms-appx:///Images/dog.png" and my code :
private void Button_Click(object sender, RoutedEventArgs e)
{
img.Source = new BitmapImage(new Uri("ms-appx:///Images/dog.png"));
}