Ho risolto così.

codice:
protected void SubmitButton_Click(Object sender, EventArgs e)
{
  HttpFileCollection uploadedFiles = Request.Files;
  String id = Request.QueryString["id"];

  for (int i = 0; i < uploadedFiles.Count; i ++)  
   {

    UploadAndResizeAnImage(uploadedFiles[i], 1800.0, 1200.0, "/images/foto/" + id, true);
  
   Response.Redirect("chiudi.asp"); 
   }  
   
}