Assuming that your DB accepts NULL values in the column:
byte[] a = null;
if (pictureBox1.Image != null)
{
MemoryStream ms = new MemoryStream();
pictureBox1.Image.Save(ms, pictureBox1.Image.RawFormat);
a = ms.GetBuffer();
ms.Close();
}
cmd.Parameters.AddWithValue("@LogoImage", a);