How to scale image from center? Windows 8.1 C#

This code scales image from left upper corner:

        Image img=new Image(){Width=150,Height=150};
        polje.RenderTransform = new CompositeTransform();
        Storyboard story = new Storyboard();

        DoubleAnimation xAnim = new DoubleAnimation();
        DoubleAnimation yAnim = new DoubleAnimation();

        xAnim.From = 0;
        yAnim.From = 0;
        xAnim.To = 1;
        yAnim.To = 0.5;

        xAnim.Duration = TimeSpan.FromMilliseconds(1000);
        yAnim.Duration = TimeSpan.FromMilliseconds(1500);

        story.Children.Add(xAnim);
        story.Children.Add(yAnim);

        Storyboard.SetTarget(xAnim, img);
        Storyboard.SetTarget(yAnim, img);

        Storyboard.SetTargetProperty(xAnim, "(UIElement.RenderTransform).(CompositeTransform.ScaleX)");
        Storyboard.SetTargetProperty(yAnim, "(UIElement.RenderTransform).(CompositeTransform.ScaleY)");

        story.Begin();