private void Form1_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
  System.IO.MemoryStream stream=e.Data.GetData(DataFormats.Dib) as System.IO.MemoryStream;
  if(stream!=null)
  { byte[] img=stream.ToArray();
    IntPtr ptr=Marshal.AllocCoTaskMem(Marshal.SizeOf(img[0])*img.Length);
    Marshal.Copy(img, 0, ptr, img.Length);
    try
    { pictureBox1.Image = TwainGui.DibToImage.WithStream(ptr);
    }
    finally
    { Marshal.FreeCoTaskMem(ptr);
    }
  }
}

private void Form1_DragDrop(object sender, System.Windows.Forms.DragEventArgs e)
{
  if(!e.Data.GetDataPresent(DataFormats.Dib) ||
     !e.Data.GetDataPresent(DataFormats.FileDrop)) return;

  try
  { // имена файлов с картинками 
    string[] fs=(string[])e.Data.GetData(DataFormats.FileDrop);
    if(fs.Length>0) // показать только первый
      pictureBox1.Image=Image.FromFile(fs[0]);
  }
  catch(Exception ex)
  { 
  }
}