如何从后台代码替换Pivot应用程序背景？

<controls:Pivot Title=\"My Application\">
<controls:Pivot.Background>
    <ImageBrush ImageSource=\"Theme1.png\" Stretch=\"Fill\"/>
</controls:Pivot.Background>

ImageBrush

<ImageBrush x:Name=\"ibPivot\" ImageSource=\"Theme1.png\" Stretch=\"Fill\"/>

BitmapImage bi = new BitmapImage(new Uri(\"mySecondImage.png\", UriKind.Relative));
ibPivot.ImageSource = bi;