//可能你的byte[] imgdata 没有得到流数据:
//here ,use byte[] b instead of byte[] imgdata
//please try the code below:byte[] b = (byte[])cmd.ExecuteScalar();
        if(b.Length > 0)
        {
            System.IO.MemoryStream stream = new System.IO.MemoryStream(b, true);
            stream.Write(b, 0, b.Length);
            
           this.pictureBox1.Image= Image.FromStream(stream);
     }