Slideshow as buttons, showing pictures when clicking

We have made a slideshow, scrolling down on the right side of the sceen. The pictures are also buttons. We would like to show a bigger picture of the small picture when the button is released. This should be placed in the center of the scene, and stay there till the next picture (button) is clicked. How do we do that? What action script command is the right one to use?
Thanks in advance!