I dont know if you have found your solution or not..
but this is how it works for me..
Code:
public static ImageSource ToImageSource(FrameworkElement obj)
{
// Save current canvas transform
Transform transform = obj.LayoutTransform;
obj.LayoutTransform = null;
// fix margin offset as well
Thickness margin = obj.Margin;
obj.Margin = new Thickness(0, 0,
margin.Right - margin.Left, margin.Bottom - margin.Top);
// Get the size of canvas
Size size = new Size(obj.Width, obj.Height);
// force control to Update
obj.Measure(size);
obj.Arrange(new Rect(size));
RenderTargetBitmap bmp = new RenderTargetBitmap(
(int)obj.Width, (int)obj.Height, 96, 96, PixelFormats.Pbgra32);
bmp.Render(obj);
// return values as they were before
obj.LayoutTransform = transform;
obj.Margin = margin;
return bmp;
}