Response.Clear();
Response.ContentType = "image/PNG";
img.Save(Response.OutputStream, ChartFormat.Png);

Response.ContentType = "image/jpeg";

Response.ContentType = "image/PNG";
using (MemoryStream ms = new MemoryStream())
{
img.Save(ms, ChartFormat.Png);
Response.OutputStream.Write(ms.GetBuffer(), 0, (int)ms.Length);
}