CONTROL TO IMAGE

hi everyone     i need some help please  .... (my eng is not very good XD )
i have to pass a webcontrol like a gridview o textbox o label ....to  image(jpg)   dinamic.
to download whit the response.addaheader

Response.ContentType = "image/jpeg" ;           
                 Response.AddHeader("Content-Disposition", "attachment;filename=ok.jpg");
 
i try a lot  of case's and nothing work...  or i have a empty image ..   
i try a ..
   StringBuilder sb2 = new StringBuilder();
        using (StringWriter sw2 = new StringWriter(sb2))
        { using (HtmlTextWriter textWriter = ne ...