为什么给按钮加背景BITMAP没效果？

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;
   PAINTSTRUCT ps;
   hInst = hInstance; // 将实例句柄存储在全局变量中

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
  
    HWND HB1=CreateWindow(_T("button"),_T(""),BS_PUSHBUTTON
        | WS_CHILD | WS_VISIBLE, 10, 15, 70, 50, hWnd,
        (HMENU)IDB_BITMAP1, NULL, NULL);
   
    HDC hdc=GetDC(HB1);
    HBITMAP bitmap=LoadBitmap (hInstance,MAKEINTRESOURCE(IDB_BITMAP1));
    HDC hdcsource = CreateCompatibleDC(NULL);//创建存放图象的显示缓冲
    SelectObject(hdcsource,bitmap); //将位图资源装入显示缓冲
    if(BitBlt(hdc,0,0,50,50,hdcsource,50,50,SRCCOPY)) MessageBox(NULL,_T("123"),_T("123"),0);//将图象显示缓冲的内容直接显示到屏幕
    ReleaseDC(HB1,hdc);
   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}