C# Parameter is not valid occur when using System.drawing

private void saveFile(HttpPostedFile hpf, string strFileName)
        {
            System.Drawing.Image myimage = System.Drawing.Image.FromStream(hpf.InputStream, false, false);
            int imageSize = hpf.ContentLength;
            hpf = null;
            colorProcessing cp = averageColor(cropImage(myimage));
            addText(myimage,cp);
            
            myimage.Save(Server.MapPath("~/PlantsPhoto/") + strFileName);
            myimage.Dispose();
            System.Threading.Thread.Sleep(imageSize / 10);
        }

        public void addText(System.Drawing.Image myimage, colorProcessing cp)
        {

            var w = myimage.Width;
            var h = myimage.Height;
            double gaugeSize = h >= w ? h / 100 : w / 100;
            double fontSize = ((gaugeSize / 10) * (10 - (gaugeSize / 10))) + 4;
            System.Drawing.StringFormat strFormat = new System.Drawing.StringFormat();
            strFormat.Alignment = System.Drawing.StringAlignment.Near;
            strFormat.LineAlignment = System.Drawing.StringAlignment.Near;
            System.Drawing.Brush b= cp.colorTone!=0?System.Drawing.Brushes.White:System.Drawing.Brushes.Black;
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(myimage);
            
            g.DrawString("xx©Copyright", new System.Drawing.Font("Tahoma", (int)fontSize), b,
                new System.Drawing.RectangleF(0, 0, w / 2, h / 2), strFormat);
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
            g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
            g.Dispose();
            myimage.Dispose();
            strFormat.Dispose();
            cp.Dispose();// = null;
            b.Dispose();
        }
        private System.Drawing.Bitmap cropImage(System.Drawing.Image img)
        {
            System.Drawing.Rectangle cropArea = new System.Drawing.Rectangle(0,0,img.Width / 2, img.Height/3);
            System.Drawing.Bitmap bmpImage = new System.Drawing.Bitmap(img);
            System.Drawing.Bitmap bmpCrop = bmpImage.Clone(cropArea,bmpImage.PixelFormat);
            //bmpCrop.Save(Server.MapPath("~/PlantsPhoto/") + "ssss.jpg");
            bmpImage.Dispose();
            //img.Dispose();
            
            return bmpCrop;
        }
        public colorProcessing averageColor(System.Drawing.Bitmap img)
        {
            System.Drawing.Bitmap MyBitmap = img; 
            System.Drawing.Rectangle Rect = new System.Drawing.Rectangle(0, 0, MyBitmap.Width, MyBitmap.Height);
            System.Drawing.Imaging.BitmapData BmpData = MyBitmap.LockBits(Rect,
                                               System.Drawing.Imaging.ImageLockMode.ReadOnly,
                                               MyBitmap.PixelFormat);


            int Size = BmpData.Stride * MyBitmap.Height;
            byte[] RGBs = new byte[Size];


            IntPtr Pointer = BmpData.Scan0;
            System.Runtime.InteropServices.Marshal.Copy(Pointer, RGBs, 0, Size);


            MyBitmap.UnlockBits(BmpData);


            // we've got all the RGB values in an array
            int Sum = 0;
            int whiteCount=0;
            for (int Index = 0; Index < RGBs.Length; Index++)
            {
                Sum += RGBs[Index];
                if (RGBs[Index]<50)
                {
                    whiteCount++;
                }
            }

            // higher Average means a lighter picture (255 = white, 0 = black)
            int Average = Sum / RGBs.Length;
            colorProcessing CPInfo = new colorProcessing();
            CPInfo.avgColor = Average;
            CPInfo.colorTone = (RGBs.Length / 15) > whiteCount ? colorProcessing.tone.White : colorProcessing.tone.Black;
            MyBitmap.Dispose();
            BmpData = null;
            RGBs = null;
            return CPInfo;
        }