Can't Apply Image through code behind in silverlight

http://www.axelerate.com/SilverlightSamples/Silverlight3D/

Is good one i got the source code from Code Plex .  then i will apply the image from Xaml page it was applied.

But when i am going to apply the image from code behind it cant work in here i am posting my code 

From Xaml 

From Code Behind 

BitmapImage bitmapImage = new BitmapImage(new Uri("Pat.jpg", UriKind.Relative));
            myImageBrush.ImageSource = bitmapImage;
            dmat.Brush = myImageBrush;

but when i am executing it cause error    Object reference not set to an instance of an object

Image path is correct ..

 for example

Xaml page 

Code Behind

private void MYbTN_Click(object sender, RoutedEventArgs e)
        {
            myBrush.ImageSource = new BitmapImage(new Uri("Pat.jpg", UriKind.Relative));
        }

its working fine .. Image will be changed 

but  i want to change image through code behind