How can I change the Gui bitmap used for background from game

We are using a mainScreenGui for backgroung images. We would like to change the bitmap after reaching check points.
How can we from the client script (game loop) change the bitmap for the mainScreenGui. Please see mainScreenGui script below.

Please Help


//--- OBJECT WRITE BEGIN ---
new GuiChunkedBitmapCtrl(mainScreenGui) {
profile = "GuiContentProfile";
horizSizing = "width";
vertSizing = "height";
position = "0 0";
extent = "800 600";
minExtent = "8 8";
visible = "1";
bitmap = "~/client/images/bgnd_01";
useVariable = "0";
tile = "0";

new fxSceneWindow2D(sceneWindow2D) {
profile = "GuiContentProfile";
horizSizing = "width";
vertSizing = "height";
position = "0 0";
extent = "800 600";
minExtent = "8 8";
visible = "1";
lockMouse = "0";
};