div.close-popup 
{
    position: fixed;
    top: 10px;
    right: 10px;
    font-size: 4em;        
    color: #fff;
    z-index:150;
}

div.container-popup {
    position: relative;
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    background: rgba(0,0,0,.8);
    z-index:10000;
}

div.container-popup div.popup {
    /*
    width: 768px;
    height: 768px;
   */
    /*
    width:100%;
    height: 100%;
    */
    max-height:768px;
    max-width:768px;
    background-position:center;
    background-size:contain;
    -moz-background-size:contain;
    -o-background-size:contain;
    -webkit-background-size:contain;
    background-repeat: no-repeat;
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    margin: auto;
    overflow:auto;
}