The best way to go about doing is is to use the MovieClip
class instead of the button class. Use the onRollover event. For
example, have a movie clip that displays the image on the second
frame. The use the actionscript: nameOfMovieClip.onRollover =
If you want to use the button class to accomplish the same
task, then you will have to create an image that you want to
display then under the "over" frame of the button, place the image.
If you want the original image to be there and display the new
image, then just add a new layer and put the image in the new layer
while keeping to original layer.