Make the following changes in script,

  function changePic() {
        var square = document.getElementsByClassName("square");
        var selectBox = document.getElementById("selectBox");
        var selectedItem = selectBox.options[selectBox.selectedIndex].value;
        square[0].style.backgroundImage  = "url("+selectedItem+")";
    }

Make the following changes in script,

  function changePic() {
        var square = document.getElementsByClassName("square");//had invalid markup
        var selectBox = document.getElementById("selectBox");
        var selectedItem = selectBox.options[selectBox.selectedIndex].value;
        square[0].style.backgroundImage  = "url("+selectedItem+")";//url should be inside quotes,added first element of the array
    }