//...
MemoryStream stream = new MemoryStream(); 
stream.Write(bin, 0, bin.GetUpperBound(0)); 
pictureBox1.Image = Bitmap.FromStream(stream); 
stream.Close();

------解决方案--------------------
以下代码用Northwind测试ok

C# code

 SqlConnection conn = new SqlConnection("Data Source=.;Initial Catalog=Northwind;User ID=sa;Password=sa");
    SqlCommand myCommand = new SqlCommand("Select top 1  Picture from Categories  order by CategoryID desc", conn);

            try
            {
                conn.Open();
                SqlDataReader myDataReader;
                myDataReader = myCommand.ExecuteReader(CommandBehavior.CloseConnection);
                if (myDataReader.Read())
                {
                    byte[] bin = (byte[])myDataReader["Picture"];



                    MemoryStream stream = new MemoryStream(bin, true);
                    stream.Write(bin, 0, bin.Length);
                    pictureBox1.BackgroundImageLayout = ImageLayout.Center;
                  
                    pictureBox1.Image = Bitmap.FromStream(stream);
                 
                    stream.Close();

                }
                conn.Close();
            }
            catch (SqlException SQLexc)
            {

            }