how to save a bitmap object(bmp) as a imge (png or jpeg) using c#

If you have a Bitmap element, simply use the Save method and pass in the appropriate image format (from the System.Drawing.Imaging namespace). It looks something like this:

public static void SavePng(this Bitmap image, string fileName)
{
  if (image == null) throw new ArgumentNullException("image");
  if (string.IsNullOrWhitespace(fileName)) throw new ArgumentException("file name has not been specified");

  image.Save(fileName, System.Drawing.Imaging.ImageFormat.Png);
}

Then all you do in your code is:

myBitmap.SavePng(_fileNameForPng);

It's as easy as that.