fred-hover.js

var appButton = document.getElementById("app-button");

appButton.addEventListener("mouseover",function() {
    this.src = "path/to/image";
});

appButton.addEventListener("mouseout",function(){
  // return stuff to default
  this.src = "path/to/image";
});

// THEN CHANGE
// <img src="http://i.imgur.com/nJj0l7L.png" style="width:141px;height:43px;">
// TO
// <img id="app-button" src="http://i.imgur.com/nJj0l7L.png" style="width:141px;height:43px;">