Resize et uploaded billede proportionalt

Hejsa alle

Hvordan kan jeg proportionalt resize et uploaded billede ved kun at angive en ny bredde?

upload_file.PostedFile.SaveAs(Server.MapPath("\\dataentry\\pic\\test.jpg"));

System.Drawing.Image image = System.Drawing.Image.FromFile(Server.MapPath("\\dataentry\\pic\\test.jpg"));

System.Drawing.Image thumbImage = image.GetThumbnailImage(Convert.ToInt32(iWidth.Text),Convert.ToInt32(iHeight.Text),null,IntPtr.Zero);

image.Dispose();

thumbImage.Save(Server.MapPath("\\dataentry\\pic\\test.jpg"), ImageFormat.Jpeg);

iWidth.Text og iHeight.Text angiver jeg i TextBox

På forhånd tak
alpapa