>When an image is received on a Web server, I need to make another copy of it in a small picture. What would be the best approach to do to obtain that?

Something like this would work (you'll obviously want to verify the image types, add some error/exception handling). I'm also assuming you're using the FileUpload control. This code could go in the Click event of your "Upload" button.

System.Drawing.Image img = System.Drawing.Image.FromStream(this.FileUpload1.PostedFile.InputStream);
System.Drawing.Image thumb = img.GetThumbnailImage(100, 100, null, IntPtr.Zero);
thumb.Save(@"C:\Sample.jpg");