@media screen and (min-width:641px){
.showimg{

        z-index:100;
        position: fixed;
        display:none;
        top:20%;
        width:700px;
        text-align:center;
        left:30%
    }
    .showimgdiv{position:relative;text-align:center;}
    .shangyige{left:0;position:absolute;top:49%;cursor:pointer;}
    .xiayige{position:absolute;top:49%;right:0;cursor:pointer;}
    #backbg{   
        width: 100%;
        height: 100%;
        background-color: #000;
        opacity: 0.4;
        filter: alpha(opacity=40);
        z-index: 10;
        display: none;
        position: fixed;
        z-index:10;
    } 
}