Draw border on detect object in image

Hi,
I am developing a web application which are based on image processing. I am not able to fix one issue in my application. I want to draw border on transparent images. i.e for a round transparent image I need to draw border round shaped. Need to detect whole image path and draw border based on that. I used something like
 

var borderColor = Color.White;
 
            var newSize = new Size(
                original.Width + borderWidth * 2,
                original.Height + borderWidth * 2);
 
            var img = new Bitmap(newSize.Width, newSize.Height);
            var g = Graphics.FromImage(img);
 
            g.Clear(borderColor);
            g.DrawImage(original, new Point(borderWidth, borderWidth));
            g.Dispose();

 
but this is not working based on transparent image.