The Image element has recently bee updated to accept url requests but it is not included in the latest release. You can either grab the latest from the development branch of the cml git repository or you can write a custom extension of the 4.1 Image element. For the latter, you would simply override the open function and initialize the greensock loader to pass the src string as a url:
img = new ImageLoader(new URLRequest(src));
You could declare an ImageViewer in CML but disable it's visibility at initialization. When you want to launch it, update the image then enable the viewer's visibility.
//access CML declared viewer in AS3
var viewer:ImageViewer = document.getElementById("iv");
var image:Image = viewer.image as Image;
viewer.visible = true;