{ .... }
 private
  function ScalePercentBmp(bitmp: TBitmap; iPercent: Integer): Boolean;
{ .... }
function TForm1.ScalePercentBmp(bitmp: TBitmap;
 iPercent: Integer): Boolean;
var
 TmpBmp: TBitmap;
 ARect: TRect;
 h, w: Real;
 hi, wi: Integer;
begin
 Result := False;
 try
  TmpBmp := TBitmap.Create;
  try
  h := bitmp.Height * (iPercent / 100);
  w := bitmp.Width * (iPercent / 100);
  hi := StrToInt(FormatFloat('#', h)) + bitmp.Height;
  wi := StrToInt(FormatFloat('#', w)) + bitmp.Width;
  TmpBmp.Width := wi;
  TmpBmp.Height := hi;
  ARect := Rect(0, 0, wi, hi);
  TmpBmp.Canvas.StretchDraw(ARect, Bitmp);
  bitmp.Assign(TmpBmp);
  finally
  TmpBmp.Free;
  end;
  Result := True;
 except
  Result := False;
 end;
end;

// Example:
procedure TForm1.Button1Click(Sender: TObject);
begin
 ScalePercentBmp(Image1.Picture.Bitmap, 33);
end;