itech9

using System.Drawing;
using System.Drawing.Imaging;

// apt-get install libgdiplus

class CGTest1 {

    public static void Main() {

        /* Image bmpImageToConvert =
           System.Drawing.Image.FromFile(
           Server.MapPath(strUploadImagePath));
         */

        Image bmpImageToConvert = Image.FromFile("images/sample3.jpg");
        Image bmpNewImage = new Bitmap(bmpImageToConvert.Width,
                                       bmpImageToConvert.Height);
        Graphics gfxNewImage = Graphics.FromImage(bmpNewImage);
        gfxNewImage.DrawImage(bmpImageToConvert,
                              new Rectangle(0, 0, bmpNewImage.Width,
                                            bmpNewImage.Height),
                              0, 0,
                              bmpImageToConvert.
                              Width,bmpImageToConvert.Height,
                              GraphicsUnit.Pixel);
        gfxNewImage.Dispose();
        bmpImageToConvert.Dispose();

        /*bmpNewImage.Save(Server.MapPath("userData/" &
          Request.QueryString("ID") & "/" & e.Item.Cells(2).Text &
          ".jpg"),ImageFormat.Jpeg)
         */

        bmpNewImage.Save("images/sample3t.png", ImageFormat.Png);
    }
}

Thanks to :

http://www.codeverge.net/ng.asp-net-forum.getting_started/converting-a-png-or-gif-to-a-jpeg