Setting the background image for an app

For a simple Windows Phone app in the MainPage.xaml file add this line right after the opening to the Grid definition:

        <Grid.Background>
            <ImageBrush ImageSource="your Background file here.png"/>
        </Grid.Background>

Note: The background.png file is not for the app it is for the Tile when your app gets pinned to the Start Screen, and it should be 173X173.