[NumLock]

Код:

type
  TButton = class(StdCtrls.TButton)
  private
    procedure CMMouseEnter(var Message: TMessage); message CM_MOUSEENTER;
    procedure CMMouseLeave(var Message: TMessage); message CM_MOUSELEAVE;
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    Image1: TImage;
...
{ TButton }

procedure TButton.CMMouseEnter(var Message: TMessage);
begin
  Form1.Image1.Picture.LoadFromFile('2.bmp');
end;

procedure TButton.CMMouseLeave(var Message: TMessage);
begin
  Form1.Image1.Picture.LoadFromFile('1.bmp');
end;

http://data.cod.ru/152238