if (pictureBox1.Tag != null)
    {
        System.Diagnostics.Process imageViewerProcess = new System.Diagnostics.Process();
        imageViewerProcess.StartInfo.CreateNoWindow = false;
        imageViewerProcess.StartInfo.FileName ="rundll32.exe";
        imageViewerProcess.StartInfo.Arguments =
                @"C:\WINDOWS\System32\shimgvw.dll,ImageView_Fullscreen "
                                   + pictureBox1.Tag.ToString();
        imageViewerProcess.Start();
    }