I would suggest that this is impractical. If someone simply runs their cursoor over the images then you get a very quick succession of load and unload image functions, which isn't good for Flash or your server.

Instead consider making it on click.

Loading the right image shouldn't be difficult, you simply need to unload the previous clip using unloadMovie(), then use loadMovie() and target the movieclip you want it to load to. If you already have your gallery in place then this change should be trivial.