transparent png over an bitmap - use in a button

Hello,

I draw a transparent png-file above an bitmap background and
it works very good.
I see a small figure above a structured bitmap.

----------------------------------------------------
//background - bitmap
     {
    CDC dcMemory;
    dcMemory.CreateCompatibleDC(pDC);

    // Select the bitmap into the in-memory DC
    CBitmap* pOldBitmap = dcMemory.SelectObject(bgBit);

    int nX = 0;
    int nY = 0;

    pDC->BitBlt(nX, nY, bmpInfo.bmWidth, bmpInfo.bmHeight, &dcMemory,
                    0, 0, SRCCOPY);

    dcMemory.SelectObject(pOldBitmap);
    }

// foreground bitmap
    {
    Gdiplus::Graphics _g(pDC->GetSafeHdc());
    Gdiplus::PointF _p(400,10);
    _g.DrawImage(imgEx, _p);
    }
----------------------------------------------------

I want to use this feature also in a button.
I have two png-files - up-state and down-state -
and I want to display them.

The MyGdiBut::DrawItem function works well,
it display the png-files in the different states.

I have already a function, which erase the background.

void MyGdiBut::InvalidateBkg()
{
    RECT rbw, rpw, rbc, r3;
    GetClientRect(&rbc);
    GetWindowRect(&rbw);
    CWnd *w1 = GetParent(), *w2 = GetParent();

    while (1)
    if (w1 = w1->GetParent())
        if (w1->GetStyle() & WS_CHILD)
        w2 = w1;
        else
        break;
    else
        break;
    w2->GetWindowRect(&rpw);

    r3.left = rbw.left - rpw.left;
    r3.top = rbw.top - rpw.top;
    r3.right = r3.left + (rbc.right - rbc.left);
    r3.bottom = r3.top + (rbc.bottom - rbc.top);
    GetParent()->InvalidateRect(&r3);
}

My problems are
- the button is not transparent
- if I call InvalidateBkg()
  in 'OnEraseBkgnd(CDC* pDC)' or
  in 'void MyGdiBut::DrawItem(LPDRAWITEMSTRUCT lpDIS)'
  the repainting never stops.

---
Thanks for help.
Frank Iversen