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type TMyThread = class(TThread)
  private
    pic1 : TGifImage;
    pic2 : TGifImage;
    PaintTo : TImage;
    p : boolean;
  public
    procedure updp;
    constructor create(startsuspended : boolean; PaintTo : TImage);
  protected
    procedure Execute; override;
  end;

type
  TForm1 = class(TForm)
    Image1: TImage;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

constructor TMyThread.create(startsuspended : boolean; PaintTo : TImage);
begin
  pic1 := TGifImage.Create;
  pic2 := TGifImage.Create;
  pic1.LoadFromFile('1.gif');
  pic2.LoadFromFile('2.gif');
//  pic1.Animate := false;
//  pic2.Animate := false;
  self.PaintTo := PaintTo;
  p := true;
  inherited Create(startsuspended);
end;

procedure TMyThread.updp;
begin
  if p then paintto.Picture.Assign(pic1)
       else paintto.Picture.Assign(pic2);
end;

procedure TMyThread.Execute;
begin
  while(true) do
    begin
    synchronize(self.updp);
    p := not p;
    sleep(1);
    end;
end;

var MyThread : TMyThread;

procedure TForm1.Button1Click(Sender: TObject);
begin
  MyThread := TMyThread.create(false,Image1);
end;