youtube-poster-frame.css

.video { position: relative; padding-bottom: 56.25%; /* 16:9 */ height: 0; }
.video img { position: absolute; display: block; top: 0; left: 0; width: 100%; height: 100%; z-index: 20; cursor: pointer; }
.video:after { content: ""; position: absolute; display: block; 
    background: url(play-button.png) no-repeat 0 0; 
    top: 45%; left: 45%; width: 46px; height: 36px; z-index: 30; cursor: pointer; } 
.video iframe { position: absolute; top: 0; left: 0; width: 100%; height: 100%; }

/* image poster clicked, player class added using js */
.video.player img { display: none; }
.video.player:after { display: none; }

youtube-poster-frame.html

<div class="video">
    <img src="poster.jpg">
    <!-- <iframe width="940" height="529" src="http://www.youtube.com/embed/rRoy6I4gKWU?autoplay=1" frameborder="0" allowfullscreen></iframe>-->
</div>

youtube-poster-frame.js

$(function() { 
    var videos  = $(".video");

        videos.on("click", function(){
            var elm = $(this),
                conts   = elm.contents(),
                le      = conts.length,
                ifr     = null;

            for(var i = 0; i<le; i++){
              if(conts[i].nodeType == 8) ifr = conts[i].textContent;
            }

            elm.addClass("player").html(ifr);
            elm.off("click");
        });
});