If I take too many screenshots the pics get black

VOID BitmapDemo (HWND hwnd)
{
 HDC hDC,                  // Handle to the display device context 
     hDCMem;               // Handle to the memory device context
  HBITMAP hBitmap,          // Handle to the new bitmap
          hOldBitmap;       // Handle to the old bitmap
  static int iCoordinate[200][4];  
  int i, j,                
      iXSrc, iYSrc,         // x and y coordinates of the source 
                            // Rectangle's upper-left corner
      iXDest, iYDest,       // x and y coordinates of the destination 
                            // Rectangle's upper-left corner
      iWidth, iHeight;      // Width and height of the bitmap

  // Retrieve the handle to a display device context for the client 
  // area of the window (hwnd). 
  if (!(hDC = GetDC (hwnd)))
   return;
  SendMessage(hwnd,WM_ERASEBKGND,(WPARAM)hDC,NULL);
  // Create a memory device context compatible with the device. 
  hDCMem = CreateCompatibleDC (hDC);

  // Retrieve the width and height of windows display elements.
  iWidth = 600;
  iHeight = 600;

  // Create a bitmap compatible with the device associated with the 
  // device context.
  hBitmap = CreateCompatibleBitmap (hDC, iWidth, iHeight);

  // Select the new bitmap object into the memory device context. 
  hOldBitmap = (HBITMAP)SelectObject (hDCMem, hBitmap);

  for (i = 0; i < 2; i++)
  {
    for (j = 0; j < 200; j++)
    {
      if (i == 0)
      {
        iCoordinate[j][0] = iXDest = iWidth * (rand () % 10);
        iCoordinate[j][1] = iYDest = iHeight * (rand () % 10);
        iCoordinate[j][2] = iXSrc = iWidth * (rand () % 10);
        iCoordinate[j][3] = iYSrc = iHeight * (rand () % 10);
      }
      else
      {
        iXDest = iCoordinate[200 - 1 - j][0];
        iYDest = iCoordinate[200 - 1 - j][1];
        iXSrc = iCoordinate[200 - 1 - j][2];
        iYSrc = iCoordinate[200 - 1 - j][3];
      }

      // Transfer pixels from the source rectangle to the destination
      // rectangle.
      BitBlt (hDCMem, 0, 0, iWidth, iHeight, hDC,  iXDest, iYDest, 
              SRCCOPY);
      BitBlt (hDC,  iXDest, iYDest, iWidth, iHeight, hDC,  iXSrc, iYSrc,
              SRCCOPY);
    }
  }

  // Select the old bitmap back into the device context.
  SelectObject (hDC, hOldBitmap);
  SaveBitmapToFile(hBitmap,"teest.bmp",0);
  // Delete the bitmap object and free all resources associated with it. 
  DeleteObject (hBitmap);

  // Delete the memory device context and the display device context.
  DeleteDC (hDCMem);
  ReleaseDC (hwnd,hDC);

  return;  
}