procedure TForm1.Button1Click(Sender: TObject);
var
 bmp: TBItmap;
 jpg: TJpegImage;
 scale: Double;
begin
 if opendialog1.execute then
 begin
 jpg := TJpegImage.Create;
 try
 jpg.Loadfromfile( opendialog1.filename );
 if jpg.Height > jpg.Width then
 scale := 50 / jpg.Height
 else
 scale := 50 / jpg.Width;
 bmp:= Tbitmap.Create;
 try
 {Create thumbnail bitmap, keep pictures aspect ratio}
 bmp.Width := Round( jpg.Width * scale );
 bmp.Height:= Round( jpg.Height * scale );
 bmp.Canvas.StretchDraw( bmp.Canvas.Cliprect, jpg );
 {Draw thumbnail as control}
 Self.Canvas.Draw( 100, 10, bmp );
 {Convert back to JPEG and save to file}
 jpg.Assign( bmp );
 jpg.SaveToFile(ChangeFileext( opendialog1.filename, '_thumb.JPG' ));
 finally
 bmp.free;
 end;
 finally
 jpg.free;
 end;
 end;
end;