document.onload = function(){
    var wMenubar = document.getElementById("wMenuBar"),
        wLogo = document.getElementById("wLogo");
    window.onscroll = function (e){
        if(window.scrollY > 50){
            wMenubar.className="small";
            wLogo.src = "logo-small.png";
        }else{
            wMenubar.className="big";
            wLogo.src = "logo.png";
        }
    }
};