Play/pause button help

I have a slideshow that plays when a button is clicked, and pauses when clicked agian. Now I want the button to change the graphic when clicked from a play picture to a pause picture and back again. I have found stuff for as2 but not really for as3 (at least nothing I could understand), so can someone give me some pointers? My as3 code for the current play button looks like this:

play_btn.addEventListener(MouseEvent.CLICK, onPlay);
function onPlay(evt:MouseEvent):void {
if (playOnClick){
timer.start();
playOnClick = false;
} else{
timer.reset();
playOnClick = true;
}