You could have a movie with the play button on a frame and the stop/pause on
another. Then in your onRelease (I prefer it to onPress. For example you press
the button and realize you didn't want to press it. Then you hold the mouse
down and move off the button. Which fires a onReleaseOutside, so don't forget
to code for that!) you can say something like this.myButtonArt.gotoAndStop(2);
Or you could give the frames labels and use those.

I have a simple MP3 player with a play and a stop button. Now what I want
to do is encapsulate the play and stop functionality in one button which
means swapping images... can any body tell me how I actually do that?

I know I probably have to do something in the event fired by the onPress but
I'm not sure what property I need to change to alter the source of an image
object.

TIA for any help...

Milke