C / C++ / MFC

hr = pActiveDesktop->SetWallpaper(L"C:\\Mina dokument\\Mina bilder\\eros15.gif", 0);

class bob
{
  public:
    int nAge;
}

class fred
{
  public:
    void LaughAtBobsAge(bob* bobclass);
}

1. Create a new CWnd-derived class, say CBackgroundWnd.
   
2. Add these two members to CBackgroundWnd:

   CBitmap m_bitmap_background;
   BITMAP m_bmInfo_background;

   along with this piece of code in CBackgroundWnd ctor:

   m_bitmap_background.LoadBitmap(IDB_BACKGROUND); // change IDB_BACKGROUND to whatever your resource-located bitmap is called.
   ::GetObject(m_bitmap_background,sizeof(BITMAP),&m_bmInfo_background);
   

   
   
3. Add a handler for WM_ERASEBKGND in CBackgroundWnd and plug this code (which draws the bitmap in tesellation mode)

   BOOL CBackgroundWnd::OnEraseBkgnd(CDC* pDC) 
   {
       CRect rect;
       GetClientRect(rect);
   
       CDC dcMem;
       dcMem.CreateCompatibleDC(pDC);
       
       HBITMAP* pBmpOld=(HBITMAP*)::SelectObject(dcMem.m_hDC,m_bitmap_background);
   
       for(int y=rect.top;y<rect.bottom;y+=m_bmInfo_background.bmHeight){
           for(int x=rect.left;x<=rect.right;x+=m_bmInfo_background.bmWidth){
               pDC->BitBlt(x,y,m_bmInfo_background.bmWidth,m_bmInfo_background.bmHeight,
                           &dcMem,0,0,SRCCOPY);
           }
       }
   
       ::SelectObject(dcMem.m_hDC, pBmpOld);
   
       return TRUE;
   }

   
   
4. Add a member of type CBackgroundWnd to your CMainFrame class, say m_wndClient.
   
5. Override CMainFrame::OnCreateClient with this:

   BOOL CMainFrame::OnCreateClient(LPCREATESTRUCT lpcs, CCreateContext* pContext) 
   {
   	if(!CMDIFrameWnd::OnCreateClient(lpcs, pContext))return FALSE;
   	m_wndClient.SubclassWindow(m_hWndMDIClient);
   	return TRUE;
   }

ShowWindow(SW_SHOW);
SetForegroundWindow();