Scaling MC's within Stage size

hey guys,
Im having a problem here. I have an MC which loads images from an xml file.
Most of them pictures are quite big (800×600) and i know some people still run that resolution. So the problem is if the browser window is smaller then those dimensions people cant see the full pic. So now my question was, does someone know how to scale them down, without stretching the pic. Thus **not: **

myMc._width = Stage.width;
myMc._height = Stage.height;

but something like http://www.manipulator.com/
where the _xscale = _yscale;

Does anyone have a clue how to write this?