$(document).ready(function(){
    $("#m_topnav a").bind("mouseover", function(){
		var oldimg=$(this).children().attr("src");
        $(this).children().attr("src", oldimg.replace(/a____/g,"b____")); 
    });
    $("#m_topnav a").bind("mouseout", function(){
		var oldimg=$(this).children().attr("src");
        $(this).children().attr("src", oldimg.replace(/b____/g,"a____")); 
    });
});