int state = 0;
//тут заголовок обработчика
ImageBrush myBrush = new ImageBrush();
if (state == 0)
            myBrush.ImageSource = new BitmapImage(new Uri("pack://application:,,,/kot.jpg", UriKind.Absolute));
else
            myBrush.ImageSource = new BitmapImage(new Uri("pack://application:,,,/kot1.jpg", UriKind.Absolute));
            this.Background = myBrush;
state = 1 - state;