I have a bunch of hardiness zones, as you can see here: http://mangoly.herokuapp.com/workroom/googlemaps/zones/

They are solid colored PNG images. I know the geo coordinates of the bottom left hand corner of all the images (they are all in the same corner). I am able to add these to the Google Maps API as groundoverlays with no problem. However I would like to make these layers clickable. So my guess is I need to convert the images into some kind of vector/geo data format. Because since each image has white space, they are all stacked on top of each other. So when I click on a layer, it will think I am choosing top most image. I need to some how convert the image into a format that make google maps understand that I only care about the actual solid color and not the white space (polygons maybe?).

Any ideas?