add text in image

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C#

Hi Every body,
here is something to add text in an image. The problem is the image file size is changed when I saves the image on harddisk. can someone guide me how I can add text in an image with transperent background.
here it goes

public static Bitmap   insertTextInImage(string Path,string Text)
{
    Bitmap myBitmap = new Bitmap(Path);
    Graphics g = Graphics.FromImage(myBitmap);
    StringFormat strFormat = new StringFormat();
    strFormat.Alignment = StringAlignment.Center;
    g.DrawString(Text, new Font("Tahoma", 10), Brushes.Black ,
        new RectangleF(0, 0, 140, 50), strFormat);

    return myBitmap;
}