var styleRules = {
    
		'img.main_nav_img' : function(element){
			element.onmouseover = function(){
			    //Check if the element is already on
				if (element.src.indexOf("_on.gif") > 0) {
				    return;
				} else {
				    element.src = element.src.replace(".gif","_on.gif"); 
				}
			};
			element.onmouseout = function(){			   
				    element.src = element.src.replace("_on.gif",".gif");			
			};
		}
	};
	
	Behaviour.register(styleRules);