How do I make a button conform to it's shape?

I am making a map like file, the objects in this map have peculiar shapes. I have each section of the map as a .png file. One example of the shapes in the map is an L shaped area that wraps around another section of the map. I wish to make it so that the map is initially gray, when one rolls over a section it appears in color, and when you click on the areas a description box appears. the problem is when I make a button the button initially makes a rectangular box where the transparency layer for the .png is and ends up causing overlapping. So how can I conform this mask like button?