C# - Use ImageFormatConverter.ConvertFromString To Convert A String Containing HTML Response To An Image?

in pageload
{
panelmain.Controls.Add(abc);
panelmain.Controls.Add(grid1);
string toexport;
toexport = RenderControl(panelmain);
ImageFormatConverter imgc = new ImageFormatConverter();
System.Drawing.Image convertedimage;
convertedimage = (System.Drawing.Image) imgc.ConvertFromString(toexport);
Response.ContentType = "image/jpg";
Response.AppendHeader("Content-Disposition", "inline;filename=tm.jpeg");
Response.BufferOutput = true;
Response.Charset = "utf-8";
Response.Write(convertedimage);
Response.End();
//form1.Controls.Add(abc);
}
public string RenderControl(Control ctrl)
{
StringBuilder sb = new StringBuilder();
StringWriter tw = new StringWriter(sb);
HtmlTextWriter hw = new HtmlTextWriter(tw);
ctrl.RenderControl(hw);
Response.Write(sb);
return sb.ToString();
}

The error is: ImageFormatConverter cannot convert from System.String.