Oh my, is there a particular reason you are using the TIFF file format? I highly recommend using PNG instead, it is lossless and compressed.
Use PngBitmapEncoder
instead, most graphics programs support PNG.
I would also recommend making it into an extension method that you can reuse throughout. Something like this:
public static class CanvasExtender
{
public static void SaveToImageFile(this Canvas canvas, string outputFile)
{
canvas.UpdateLayout();
var bitmap = new RenderTargetBitmap(canvas.ActualWidth, canvas.ActualHeight, 96d, 96d, PixelFormats.Pbgra32);
bitmap.Render(canvas);
var encoder = new PngBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(bitmap));
using(var outputStream = File.Create(outputFile))
encoder.Save(outputStream);
}
}