dialog box fails to re-paint

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MFC

Hello,

Please excuse the code below. I'm just hacking at the moment.
I have a question, essentially the below code displays 3 images.
The 1st image displays the background, and the remaining 2 smaller images are simple logos. It renders fine, however when I drag the dialog box. It fails to re-paint the background?

Could somebody be so kind and enlighten me on this problem and how to resolve it. My memory is indeed bad.

Thanks,
Garyc

 void CtesterDlg::OnPaint()
   {
   	CPaintDC dc( this ); // Device context for painting
         CBitmap bmp, *poldbmp;
         CDC memdc;
        	  // Load the bitmap resource
		 bmp.LoadBitmap(IDB_BITMAP1);
         // Create a compatible memory DC
         memdc.CreateCompatibleDC( &dc );
         // Select the bitmap into the DC
         poldbmp = memdc.SelectObject( &bmp );
         // Copy (BitBlt) bitmap from memory DC to screen DC
         dc.BitBlt(0, 0,2000,4000, &memdc, 0, 0, SRCCOPY );   

         // Copy (BitBlt) bitmap from memory DC to screen DC
    
       
         memdc.SelectObject( poldbmp );
		   // Load the bitmap resource
		 bmp.LoadBitmap(IDB_BITMAP2);
         // Create a compatible memory DC
         memdc.CreateCompatibleDC( &dc );
         // Select the bitmap into the DC
         poldbmp = memdc.SelectObject( &bmp );
         // Copy (BitBlt) bitmap from memory DC to screen DC
         dc.BitBlt(20,320,400,400, &memdc, 0, 0, SRCCOPY );
		  memdc.SelectObject( poldbmp );
		   // Load the bitmap resource
		 bmp.LoadBitmap(IDB_BITMAP3);
         // Create a compatible memory DC
         memdc.CreateCompatibleDC( &dc );
         // Select the bitmap into the DC
         poldbmp = memdc.SelectObject( &bmp );
         // Copy (BitBlt) bitmap from memory DC to screen DC
         dc.BitBlt(550,320,450,6000, &memdc, 0, 0, SRCCOPY );
}