// Example in C#,should be quite alike in ASP.NET
// Assuming filename as the uploaded file
using ( Image bigImage = new Bitmap( filename ) )
{
   // Algorithm simplified for purpose of example.
   int height = bigImage.Height / 10;
   int width = bigImage.Width / 10;

   // Now create a thumbnail
   using ( Image smallImage = image.GetThumbnailImage( width,height,new Image.GetThumbnailImageAbort(Abort),IntPtr.Zero) )
   {
      smallImage.Save("thumbnail.jpg",ImageFormat.Jpeg);
   }
}