Dynamic Mask

Thanks for your advice. I ended up drawing the mask and loading the image to be masked from the root folder. This worked. 

Maybe not the cleanest code but I'm still learning some of the basics:

createjs.Ticker.addEventListener("tick", tick);

var BG = new createjs.Bitmap("Background.jpg");
stage.addChild(BG);
var Mask = new createjs.Shape();
Mask.graphics.mt(1, 40).lt(1, 280).lt(280, 390).lt(280, 140).cp();
BG.mask = Mask;
stage.addChild(Mask);

function tick(event) {
stage.update();
}

Handle = new lib.handle();
stage.addChild(Handle);
Handle.on ("pressmove", moveMask);

function moveMask(e) {
var p = stage.globalToLocal(e.stageX, e.stageY)
e.currentTarget.x = p.x;
exportRoot.XOutput.text = Handle.x;
Mask.x = Handle.x;
}