I tried following steps and it worked for saving rotated image with WritableBitmap,
Step 1) Placed Image control inside StackPanel control as mentioned below,
<StackPanel x:Name="ContentPanel" Grid.Row="1" Background="Black" Height="400" Width="400" Margin="0, 50, 0, 50" >
<Image x:Name="img" RenderTransformOrigin="0.5, 0.5" CacheMode="BitmapCache" Grid.Row="1" Stretch="UniformToFill" Height="400" Width="400" >
<Image.RenderTransform>
<CompositeTransform x:Name="transform" />
</Image.RenderTransform>
</Image>
</StackPanel>
Step 2) Used following code to save WitableBitmap using StackPanel "ContentPanel" as I added Image control on ContentPanel,
IsolatedStorageFile isolatedStorage = IsolatedStorageFile.GetUserStoreForApplication();
if (isolatedStorage.FileExists(filePath))
isolatedStorage.DeleteFile(filePath);
var fileStream = isolatedStorage.CreateFile(filePath);
WriteableBitmap wb = new WriteableBitmap(ContentPanel, null);
wb.SaveJpeg(fileStream, wb.PixelWidth, wb.PixelHeight, 0, 100);
fileStream.Close();