string deviceInfo = "<DeviceInfo><OutPutFormat>PNG</OutPutFormat></DeviceInfo>";
string format="image";
byte[] bytes = ReportView1.ServerReport.Render(format,deviceInfo,out mimeType,out encoding,out fileNameExtension,out streamids,out warnings);
FileStream fs = new FileStream("hi.png",FileMode.Creat);
fs.Write(bytes,0,bytes.Length);
fs.Close();