Resize Image of any size to fixed dimension using C# ASP.Net web form

I wrote an article on using jCrop with dynamically-resized uploaded images, which seems to be what you're needing.

If you're looking for an open-source ASP.NET control that does it for you, check out cropimage.net.

Want to going to by programmatically than you can try this :

if you are using file upload for upload images

    string path = Path.GetFileName(fileuploaderID.PostedFile.FileName); 
    ConvertThumbnails(width, height, fileuploaderID.FileBytes, path);

your function

       public void ConvertThumbnails(int width, int height, byte[] filestream, string path)
      {
        // create an image object, using the filename we just retrieved

        var stream = new MemoryStream(filestream);
        System.Drawing.Image image = System.Drawing.Image.FromStream(stream);

         try
              {  
                int fullSizeImgWidth = image.Width;
                int fullSizeImgHeight = image.Height;
                float imgWidth = 0.0F;
                float imgHeight = 0.0F;
                imgWidth = width;
                imgHeight = height;

                Bitmap thumbNailImg = new Bitmap(image, (int)imgWidth, (int)imgHeight);
                MemoryStream ms = new MemoryStream();
                // Save to memory using the Jpeg format
                thumbNailImg.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
                // read to end
                byte[] bmpBytes = ms.GetBuffer();
                item.Attachments.Add(path, bmpBytes);
                thumbNailImg.Dispose();
                ms.Close();

        }
        catch (Exception)
        {
            image.Dispose();
        }
    }