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ImageX.Picture.Bitmap.LoadFromResource                    

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procedure TForm1.Image5MouseEnter(Sender: TObject);
begin
Image5.Picture.LoadFromFile('dark.png');
end;

procedure TForm1.Image5MouseLeave(Sender: TObject);
begin
Image5.Picture.LoadFromFile('null.png');
end;                    

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uses
  PNGImage;

var
  Res: TResourceStream;
  Png: TPNGImage;
begin
  Res := TResourceStream.Create(HInstance, 'PngImageResName', 'PNG');
  Png := TPNGImage.Create;
  Png.LoadFromStream(Res);
  Image1.Picture.Graphic := Png;
  Res.Free;                    

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Damit das beschleunigt ist, solltest du die Bilder in FormCreate in zwei TPNGImage-Variablen laden und dann nur noch das richtige zuweisen.
Warum hat user profile iconturboPASCAL ja gerade geschrieben.                    

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type
  TForm1 = class(TForm)
  ...
  public
    Png1, Png2: TPNGImage;
  end;

...

procedure TForm1.FormCreate(Sender: TObject);
var
  Res: TResourceStream;
begin
  Res := TResourceStream.Create(HInstance, 'PngImageResName1', 'PNG');
  Png1 := TPNGImage.Create;
  Png1.LoadFromStream(Res);
  Res.Free;

  Res := TResourceStream.Create(HInstance, 'PngImageResName2', 'PNG');
  Png2 := TPNGImage.Create;
  Png2.LoadFromStream(Res);
  Res.Free;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FreeAndNil(Png1);
  FreeAndNil(Png2);
end;

procedure TForm1.ImageXMouseEnter(Sender: TObject);
begin
  ImageX.Picture.Graphic := Png1;
end;

procedure TForm1.ImageXMouseLeave(Sender: TObject);
begin
  ImageX.Picture.Graphic := Png2;
end;