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const imgPath : string = 'C:\Users\user\Pictures\temp.png';

{...}

procedure TForm1.OpenPictureDialog1SelectionChange(Sender: TObject);
var png1, png2: TPNGObject;
begin
  Canvas.Rectangle(0, 0, ClientWidth, ClientHeight);
  png1 := TPNGObject.Create;
  png1.LoadFromFile(OpenPictureDialog1.FileName);
  Canvas.StretchDraw(Rect(0, 0, ClientWidth div 2, ClientHeight), png1);
                                             
  {--------------------------------------------------------------------------}

    //COLOR_RGB: schwarzer Hintergrund anstatt Transparenz
    //COLOR_RGBALPHA: komplett weißes Bild
  png2 := TPNGObject.CreateBlank(COLOR_RGB, 16, ClientWidth div 2, ClientHeight);
  //png2.CreateAlpha;

  png1.Draw(png2.Canvas, Rect(0, 0, png2.Width, png2.Height));

  //Canvas.StretchDraw(Rect(ClientWidth div 2, 0, ClientWidth, ClientHeight), png2);
  png2.Draw(Canvas, Rect(ClientWidth div 2, 0, ClientWidth, ClientHeight));

  png2.SaveToFile(imgPath);
  
  png1.Free;
  png2.Free;
end;