Quick and dirty solution:

1) Convert the image to a button,
2) Copy the Up state to the Over state
3) Set the transparency to 0% on the image in the Over State
4a) If you need a transition from 50% to 0%, before step 1 above, copy the image to a movie clip and create a multiframe clip, w/ image on first frame starting at 50% and image on last frame at 0% transparency. Don't forget to put a stop() on the last frame or you will have a cycling image.
4b) After step 1) above, create a blank Over state and cut and paste the clip from the stage to the button's Over state; disregard steps 2) and 3).
5) Create a Down state.