please, how activate button hero_btn_id covered transparent image top_image_id? or more precisely, how disable layer top_image_id? :)
<div id="back_image_id">
   <a id="hero_btn_id" href="#"></a>
</div>
<div id="top_image_id"></div>
Example http://jsfiddle.net/3ebdb/
Complete example https://dl.dropboxusercontent.com/u/55504690/examples/web/index.html
 
    