just make the contaier to have application rect instead client rect, so it will fill the whole screen. then in your draw just use the rect passed to it to fill the screen with your image.
Here's how you draw them [from the Draw(TRect aRect) function where aBitmap is of type CFbsBitmap]:
CWindowGc& gc = SystemGc();
(1) TRect rect = Rect(); // to stretch/shrink to full client area
gc.DrawBitmap(rect, aBitmap);
(2) TRect rect = Rect();
rect.Resize(0.5, 0.5) // (made up function) to draw to half client area
gc.DrawBitmap(rect, aBitmap);
(3) TRect rect = Rect();
rect.Resize(2, 2) // (made up function) to draw to double client area - needs scrollbars
gc.DrawBitmap(rect, aBitmap);
(4) // to display the bitmap at full size regardless of screensize use BitBlt
// Sorry, haven't used this so can't advise any further on it
gc.BitBlt(aRect.iTl, aBitmap, redrawRect);