var myrules = {
    'img.button' : function(el){
        if(el.name != nav && el.name != subnav){
            el.onmouseover = function(){
                var filename = this.src
                filename = filename.replace(".png","_over.png")
                this.src = filename;
            }
            
            el.onmouseout = function(){
                var filename = this.src
                filename = filename.replace("_over.png",".png")
                this.src = filename;
            }
        }
    }
};

Behaviour.register(myrules);