uses
 JPEG;
procedure JPEGtoBMP(const FileName: TFileName);
var
 jpeg: TJPEGImage;
 bmp: TBitmap;
begin
 jpeg := TJPEGImage.Create;
 try
  jpeg.CompressionQuality := 100; {Default Value}
  jpeg.LoadFromFile(FileName);
  bmp := TBitmap.Create;
  try
  bmp.Assign(jpeg);
  bmp.SaveTofile(ChangeFileExt(FileName, '.bmp'));
  finally
  bmp.Free
  end;
 finally
  jpeg.Free
 end;
end;

{
 CompressionQuality (default 100):
 Set a value between 1..100, depending on your need of quality and
 image file size. 1 = Smallest file size, 100 = Best quality.
}