private void ResizeAndSave(Stream imgStr,int Width, int Height,string FileName)
{
  //creo il bitmap dallo stream
  System.Drawing.Image bmpStream=System.Drawing.Image.FromStream(imgStr);
  //creo un nuovo bitmap ridimensionandolo
  Bitmap img=new Bitmap(bmpStream,new Size(Width,Height));
  //salvo l'immagine ridimensionata
  img.Save(FileName,System.Drawing.Imaging.ImageFormat.Jpeg);

  Response.Write("fatto!");
}

FileStream file=new FileStream(nomefile,FileMode.Open);

ResizeAndSave(file,100,50,nuovonome);

file.Close();

System.Drawing.Image bmpStream=System.Drawing.Image.FromStream(imgStr);

Bitmap img=new Bitmap(bmpStream,new Size(Width,Height));

img.Save(FileName,System.Drawing.Imaging.ImageFormat.Jpeg);