convert byte[] of jp2 to jpg file

We don't have anything built in .Net to do this but, You can use FreeImage which is a free library that can do this.

Here is an Example on doing this.

FIBITMAP dib = FreeImage.LoadEx("test.jp2");
//save the image out to disk    
FreeImage.Save(FREE_IMAGE_FORMAT.FIF_JPEG, dib, "test.jpg", FREE_IMAGE_SAVE_FLAGS.JPEG_QUALITYNORMAL);
//or even turn it into a normal Bitmap for later use
Bitmap bitmap = FreeImage.GetBitmap(dib);

For converting from a stream of bytes u can try this:

byte[] myByte = new byte[10];
MemoryStream theMemStream = new MemoryStream();
theMemStream.Write(myByte, 0, myByte.Length);
FreeImageBitmap fbm = FreeImageBitmap.FromStream(theMemStream);
fbm.Save("text.jpg",FREE_IMAGE_STREAM.FIF_JPEG);