The problem is that new Bitmap(ms) is going to read the data from the stream's current position. And if the stream is currently positioned at the end of the data, it's not going to be able to read anything, hence the problem occurs. Try with below code:
string name = cbCust_Name.SelectedValue.ToString();
SqlConnection cnn = new SqlConnection(conString);
cnn.Open();
SqlCommand command = new SqlCommand("Select Cust_Image from Photo where Cust_Name= '" + name + "'", cnn);
byte[] image = (byte[])command.ExecuteScalar();
cnn.Close();
MemoryStream ms = new MemoryStream(image);
ms.Seek(0, SeekOrigin.Begin);
Bitmap bitmap = new Bitmap(ms);
pictureBox1.Image = bitmap;
Suggestions: Please try to use Parameterised query when you are using inline SQL query. It helps to avoid SQL injection. Secondly close the connection and memory stream object properly.