Lord_Dantes

<img id="image" src="Null_Image.png" />
<select id="CarList">
    <option value="Null_Image.png">No Car</option>
    <option value="volvo.png">Volvo</option>
    <option value="audi.png">Audi</option>
</select>

function setCar() {
    var img = document.getElementById("image");
    img.src = this.value;
    return false;
}
document.getElementById("CarList").onchange = setCar;

<script src="jquery.js"></script>
<script src="main.js"></script>

var oldScrollY = 0;
        var div = document.getElementById("masthead");
        var footer = document.getElementById("footer");
        window.onscroll = function() {
          var scrolled = window.pageYOffset || document.documentElement.scrollTop;
          var dY = scrolled - oldScrollY;
          
          if ( dY > 0 ){
            div.className = "site-header site-header-hide";
            footer.className = "footer footer--active";
          } else {
            div.className = "site-header";
             footer.className = "footer";
          }
          
          oldScrollY = scrolled;
        }