上传图片生成缩略图

{
                this.labIamge.Visible = true;
                labIamge.Text = "";
                if (imageUpload.PostedFile.FileName == "")
                {
                    Response.Write(ccObj.MessageBox("要上传的文件不允许为空！"));
                    return;
                }
                else
                {
                    string filePath = imageUpload.PostedFile.FileName;//获取上传文件的路径
                    string filename = filePath.Substring(filePath.LastIndexOf("\\") + 1);
                    string fileEx = filePath.Substring(filePath.LastIndexOf(".") + 1);
                    string serverpath = Server.MapPath(@"~\Images\ftp\") + filename;
                    string relativepath = @"~\images\ftp\" + filename;
                    string strSql = "select * from tb_Image where ImageName='" + filename + "'";
                    DataTable dsTable = dbClass.GetDataSetStr(strSql, "tbImage");
                    if (dsTable.Rows.Count > 0)
                    {
                        Response.Write(ccObj.MessageBox("该图片已存在！"));
                    }
                    else
                    {
                        //判断图片格式
                        if (fileEx == "jpg" || fileEx == "bmp" || fileEx == "gif")
                        {
                            //生成缩略图
                            System.Drawing.Image image, newimage;
                            image = System.Drawing.Image.FromFile(filePath);
                            System.Drawing.Image.GetThumbnailImageAbort callb = null;
                            newimage = image.GetThumbnailImage(67, 90, callb, new System.IntPtr());
                            //把缩略图保存到指定的虚拟路径
                            newimage.Save(serverpath);
                            //释放image对象占用的资源
                            newimage.Dispose();
                            image.Dispose();
                            imageUpload.PostedFile.SaveAs(serverpath);
                            //将图片的信息保存在数据库中
                            string strAddSql = "insert into tb_Image(ImageName,ImageUrl)";
                            strAddSql += "Values('" + filename + "','" + relativepath + "')";
                            SqlCommand myCmd = dbClass.GetCommandStr(strAddSql);
                            dbClass.ExecNonQuery(myCmd);
                            dlImageBind();
                            Response.Write(ccObj.MessageBox("上传成功！"));
                        }
                        else
                        {
                            Response.Write(ccObj.MessageBox("上传的图片扩展名错误！"));
                        }
                    }
                }
            }
这段代码是上传图片时生成缩略图的，但是报错文件未找到实例，是什么原因啊？
帮忙看一下吧