uses jpeg;
procedure TForm1.Button1Click(Sender: TObject);
var
bmp : TImage;
jpg : TJpegImage;
begin
bmp := TImage.Create(nil);
jpg := TJpegImage.Create;
bmp.picture.bitmap.LoadFromFile ( 'c:\picture.bmp' );
jpg.Assign( bmp.picture.bitmap );
jpg.SaveToFile ( 'c:\picture.jpg' );
jpg.Free;
bmp.Free;
end;