为啥图片加载不出来

为什么图片加载不出来?
分裂图片--一张照片放进BitmapImage里面然后利用WriteableBitmap将图片分成4块

<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Image Name="imgUL" Grid.Row="0" Grid.Column="0" Margin="2"/>
            <Image Name="imgUR" Grid.Row="0" Grid.Column="1" Margin="2"/>
            <Image Name="imgLL" Grid.Row="1" Grid.Column="0" Margin="2"/>
            <Image Name="imgLR" Grid.Row="1" Grid.Column="1" Margin="2"/>
        </Grid>




 int picturewidth;
        int pictureheight;
        public MainPage()
        {
            InitializeComponent();
            picturewidth =((int)ContentPanel.ActualWidth)- 8;
            pictureheight = ((int)ContentPanel.ActualHeight) - 8;
            // 用于本地化 ApplicationBar 的示例代码
            //BuildLocalizedApplicationBar();
            
        }

        protected override void OnManipulationStarted(ManipulationStartedEventArgs args)