Flash and PNG

Hi,

I have one question for you guys.

I’m currently building a project where I have a map, the map has individual pieces (cities) where the user can rollover with the cursor on each city.

To be able to do that, I saved each city as png with background transparency, however, when I import to flash and make it active, the city stays a png with the transparency but becomes a square, which means that when the user rollover the transparent square part activates it.

What I need is to know how to use only the form of the city in flash.

(See image below).

Thanks in advance.