Hmm I'm not 100% what your problem is due to a lack of info. I would however try the following:
1. Add a trace to the rollOver - ie
on(rollOver){
moviePic1._visible = false;
trace ("test");
}
Now test, if it flashes up 'Test' you know the rollOver is working and the problem is probably an error in instance name your calling.
If not check..
2. Your pictures are 'movie clips' NOT graphics (AS can only control 'movies' not graphics)
If you don't get the 'Test' pop-up:
3. Check the top rectangle is a BUTTON
4. That you placed the actionscript IN the button. This is maybe a hard concept to grasp maybe but in Flash you can place actionscript on the timeline, on movieclips, on buttons, and then inside timelines of movieclips, basically you can add code anywhere and on large projects it can get messy and confusing quickly if you dont plan properly!
But basically for that code to work it needs to be ON the button symbol. The easiest way to do is press F9 whilst you have the buttons selected/highlighted. Then type in code.
There is no start/stop code as you are not using the timeline but for want of a better description - Flash's internal timing. Whilst this movie just sits on Frame1 doing nothing the code in the button constantly monitors for a rollOver.
5. Your best bet is probably to download my version and look through and see what you have done different etc..
ps.. Try out Fireworks for designing websites.. It got a nice mix of vector & bitmap tools - I have a web design studio and we all design in Fireworks & then cut up and build CSS/XHTML/PHP sites by hand.. Flash is good for bits but not whole sites IMHO!
Good luck..