Use Custom Background Image for Application-为自己的Windows Phone程序使用自定义背景图片

If you want to use a custom image for your application background, you can set this as follows:

<phone:PhoneApplicationPage >
...
<Grid x:Name="LayoutRoot">
<Grid.Background>
<ImageBrush ImageSource="/Images/YourImage.jpg"></ImageBrush>
</Grid.Background>
...
</Grid>
</phone:PhoneApplicationPage>

Some things to note:

Make sure the Image has the Build Action set to Content.
Make sure the Images folder is at the application root level.
Having your own background and colour combination will have the same look across bothLight and
Dark themes.

原文链接：http://amitchandnz.wordpress.com/2011/09/26/use-custom-background-image-for-application/