This solution is HTML and CSS based using z-index and hover, which works if JS is disabled or the video isn't yours (since you can add a thumbnail in YouTube).
<style>
.videoWrapper {
position: relative;
padding-bottom: 56.25%;
}
.videoWrapper .video-modal-poster img {
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
z-index: 10;
}
.videoWrapper .video-modal-poster:hover img {
z-index:0;
}
.videoWrapper iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: 1;
}
</style>
<div class="videoWrapper">
<a href="#" class="video-modal-poster">
<img alt="" src="" width="353" height="199" />
<iframe width="353" height="199" src="https://www.youtube.com/embed/" frameborder="0" allowfullscreen="allowfullscreen"></iframe>
</a>
</div>