private void Convert_Click(object sender, EventArgs e)
        {
            MemoryStream MemStream = new MemoryStream();
        // YourPicture = Your PictureBox
            YourPicture.Image.Save(MemStream, System.Drawing.Imaging.ImageFormat.Png);
            byte[] pngBase = MemStream.ToArray();
            MemStream.Close();
        //pngBase64 = Your TextBox
            pngBase64.Text = Convert.ToBase64String(pngBase);
        }