Automatically scale bitmap to window?

So I'm putting finishing touches on my first rudimentary "Main Menu", and I've run across a small problem. I want to set a background image for the main menu using a GuiBitmapCtrl object, and I can get it to auto-size to the window on startup, but I cant seem to get it to resize with the window AND automatically scale itself to the window when the game begins. Here's what I've done:

%bg = new GuiBitmapCtrl()
{
HorizSizing = "";//cannot figure out what to set these to to get this right
VertSizing = "";
profile = MMGUI;
//extent = Canvas.getExtent(); //doesn't work, canvas does not start in the proper resolution
extent = getwords($pref::video::windowedres, 0, 1); // is there a way to do this with TAML?
bitmap = "./images/space.png";
cliptoparent = true;
};

If anyone has an idea, I would like to finish this up so I can work on some type of documentation of what I've learned to help the Gui documentation with everyone else.