Drawing directly on the screen

Yesterday, I asked a question about drawing directly to the screen. I was told to use the GetDesktopWindow() function, but all I could get was the following:

mainWnd = m_pMainWnd->GetDesktopWindow();
MyDC = mainWnd->GetWindowDC();
MyDC->TextOut(20, 20, "A text");

(I put it to the end of the InitInstance function)

This displayed a text on the screen above the windows. (which doesn't always appeared normally!)

My question is: how could I display a bitmap somewhere, and update it dinamically? Like when a window is maximized, it wouldn't overwrite the bitmap on the screen?

I saw some apps which did this, but I dunno how to. (like MagicSpell for ICQ)

Please help!!!

Thanks: Joe :)