img.imagen-previa { 
    display: block; 
    left: 0; 
    bottom: 0;
    margin: auto; 
    max-width: 100%; 
    width: 100%; 
    position: absolute; 
    right: 0; 
    top: 0; 
    height: auto 
}
div.youtube-play { 
    height: 64px; 
    width: 64px; 
    left: 50%; 
    top: 50%; 
    margin-left: -36px; 
    margin-top: -36px;
    opacity:0.7;
    position: absolute; 
    background:   url("https://cdn2.iconfinder.com/data/icons/social-icons-color/512/youtube-64.png") no-repeat; 
}
div.youtube-play:hover{
    opacity:1; 
}
#youtube-iframe { 
    width: 100%; 
    height: 100%; 
    position: absolute; 
    top: 0; 
    left: 0; 
}