Why the image is displayed only after I minimize the Window ?

    case WM_PAINT:
    {
        hdc = BeginPaint(hwnd,&paintSt);
        temphdc = hdc;
        GetClientRect(hwnd,&aRect);
        if(hBitmap!=NULL)
        {               
            HDC memDC = CreateCompatibleDC(hdc);
            if(memDC!=NULL)
            {
                BITMAP bmp;
		GetObject(hBitmap,sizeof(bmp),&bmp);
		SelectObject(memDC,hBitmap);
		SetStretchBltMode(hdc,HALFTONE);
		
                StretchBlt(hdc,0,0,aRect.right,aRect.bottom,
	        memDC,0,0,bmp.bmWidth,bmp.bmHeight,
		SRCCOPY);
		
                // Delete the BITMAP structure
		DeleteObject(&bmp);
		//ReleaseDC(hwnd,memDC);
		DeleteDC(memDC);  
					    
                SendMessage(picBoxCrop,STM_SETIMAGE,NULL,NULL);
            }
        }           
        // the code for painting 
        EndPaint(hwnd,&paintSt);
    }
    break;
..... now here is the code from the proc of picBoxCrop
LRESULT CALLBACK cropWndProc(HWND hwnd,UINT msg, WPARAM wParam, LPARAM lParam)
{
	static HDC hdc;
	PAINTSTRUCT paintSt;
	RECT aRect;
	switch(msg)
	{
		case WM_PAINT:
		{
			hdc = BeginPaint(hwnd,&paintSt);
			GetClientRect(hwnd,&aRect);
				// The code 
			if(hBitmap!=NULL)
			{
				HDC memDC = CreateCompatibleDC(hdc);
				if(memDC!=NULL)
				{
					SelectObject(memDC,hBitmap);
				     if(StretchBlt(hdc,0,0,aRect.right,aRect.bottom,
                                             memDC,0,0,100,100,SRCCOPY)==false)
				     {
				       MessageBox(NULL,"StretchBlt()  
                                       failed...","Error!! Bad Programming.",MB_OK);
					}
				}
				else
				MessageBox(NULL,"memDC of cropDisp cant be 
                                created","Error!! Bad Programming.",MB_OK);
				
                                DeleteDC(memDC);
			}			
			EndPaint(hwnd,&paintSt);
		}
		break;
		case STM_SETIMAGE:
			{
				InvalidateRect(hwnd,&aRect,true);
			}
			break;
		case WM_CREATE:
			{}break;	
		case WM_ERASEBKGND:
			{
				InvalidateRect(hwnd,&aRect,true);
			}break;
		default:
		 return DefWindowProc(hwnd,msg,wParam,lParam);
	}
	return 0;
}