[Decoding]

Код:

// так
uses
  ..., JPEG;

procedure TForm1.Button1Click(Sender: TObject);
var
  MyJPEG : TJPEGImage;
  MyBMP : TBitmap;
begin
   MyJPEG := TJPEGImage.Create;
   with MyJPEG do
   begin
      LoadFromFile( 'YourJpegHere.JPEG' );
      MyBMP := TBitmap.Create;
      with MyBMP do
      begin
         Width := MyJPEG.Width;
         Height := MyJPEG.Height;
         Canvas.Draw( 0, 0, MyJPEG );
         SaveToFile( 'YourBmpHere.BMP' );
         Free;
      end;
      Free;
   end;
end;

// или так
uses
  ..., JPEG;

procedure TForm1.Button1Click(Sender: TObject);
var
  MyBMP: TBitmap;
  MyJPEG: TJPEGImage;
begin
   MyBMP := TBitmap.Create;
   MyJPEG := TJPEGImage.Create;

   MyJPEG.LoadFromFile( 'YourJpegHere.JPEG' );
   MyBMP.Assign( MyJPEG );
   MyBMP.SaveToFile( 'YourBmpHere.BMP' );

   MyJPEG.Free;
   MyBMP.Free;
end;