procedure JpgFileToBmpFile(SourceJPG, DestinationBMP: string);
var
  JpgWidth, JpgHeight, JpgSubsamp: integer;
  DecompressBuf: PByte;
  JpgMS: TMemoryStream;
  DecompressorHandle: TJHandle;
begin
  DecompressorHandle := TJ.InitDecompress;

  // load jpg
  JpgMS := TMemoryStream.Create;
  JpgMS.LoadFromFile(SourceJPG);

  // get jpg width and height
  TJ.DecompressHeader2(DecompressorHandle, JpgMS.Memory, JpgMS.Size, @JpgWidth, @JpgHeight, @JpgSubsamp);

  DecompressBuf := GetMemory(JpgWidth * JpgHeight * 3);
  if DecompressBuf <> nil then
    begin
      // decompress jpg to mem buffer
      TJ.Decompress2(DecompressorHandle, JpgMS.Memory, JpgMS.Size, DecompressBuf, JpgWidth, 0 , JpgHeight, TJPF_RGB ,0);
      // save mem buffer to BMP file
      TJ.SaveImage(PAnsiChar(UTF8Encode(DestinationBMP)), DecompressBuf, JpgWidth, 0 , JpgHeight, TJPF_RGB, 0);

      FreeMemory(DecompressBuf);
    end;

  JpgMS.Free;
  TJ.Destroy(DecompressorHandle);
end;