http://jsfiddle.net/bdjzp1ye/

<body>
    
    <div>     <img class="top-img" src="http://placekitten.com/560/315" alt="">
    </div>

 <p class="hidden"><iframe class="video" width="560" height="315" src="http://www.youtube.com/embed/H93n-k3SkiQ?rel=0&autoplay=1" frameborder="0" allowfullscreen></iframe></p>
</body>
div {
  height:0px;
}

p.hidden {
 display:none; 
}

p {
    height:100px;   
}
$("body").on("mouseenter", "div", function(){
     $("p").removeClass("hidden");
}).on("mouseleave", "div", function(){
    $("p").addClass("hidden");
});