Need to convert all image types to JPEG using asp.net with C#

You cannot convert them during the upload. You have to let the user upload the image file, then you can try to load it into a Bitmap object and resave the image in a new format.

I'ved used code similar to this to do what you're describing before:

public static class ImageExtensions
{

    public static byte[] ToByteArray(this Bitmap sourceImage, ImageFormat format)
    {
        if (sourceImage == null)
            return null;

        using (MemoryStream targetStream = new MemoryStream())
        {
            sourceImage.Save(targetStream, format);
            targetStream.Close();

            return targetStream.ToArray();
        }
    }

    public static Bitmap ToBitmap(this byte[] bytes)
    {
        if (bytes == null)
            return (Bitmap)null;

        using (MemoryStream targetStream = new MemoryStream(bytes))
        {
            return new Bitmap(targetStream);
        }
    }

}

You can convert a Bitmap to an array of bytes in any image format you want. Then it's a simple matter to convert the array back to a Bitmap.