There is no property in ActionScript 3 called "flipImage", but there is one easy way to do it.
In AS3, to flip an image horizontally, go like so:
var flipper:MovieClip = new MovieClip();
//Load your image into the flipper MC
flipper.scaleX = -1;
This works because you're actually resizing the image beyond zero, literately turning it inside out.
PS: This will not work with components, like the UILoader.