How to fade groups of images or multiple images at the same time?

Use TweenMax for this.

the syntax is like this:

var arrayOfDisplayObjects:Array = new Array();

//with arrayOfDisplayObjects.push(DisplayObject)

//you can put any DisplayObject in the Array and when you want to fade them call this

TweenMax.allTo(arrayOfDisplayObjects,1, { autoAlpha:0} );

//to fade them in 1 second to alpha=0 and when reaching the zero make them invisible (to save processing power)