uses NetEncoding,PNGImage;

procedure TForm1.FormCreate(Sender: TObject);

var

  LInputStream: TFileStream;

  LOutputStream: TMemoryStream;

  Png: TPNGImage;

begin

  LInputStream := TFileStream.Create('C:\Temp\PlateImage.txt', fmOpenRead or fmShareDenyWrite);

  try

    LOutputStream := TMemoryStream.Create;

    try

      TNetEncoding.Base64.Decode(LInputStream, LOutputStream);

      ShowMessage(LOutputStream.Size.ToString);

      LOutputStream.Position := 0;

      Png := TPNGImage.Create;

      try

        Png.LoadFromStream(LOutputStream);

        Image1.Picture.Assign(Png);

      finally

        Png.Free;

      end;

    finally

      LOutputStream.Free;

    end;

  finally

    LInputStream.Free;

  end;

end;