.aside{
    width:50%;
    float:left;
    background-repeat:no-repeat;
    text-align:center
}

#contactar_dsk{
    display: inline-block;
}

#contactar_mob{
    display: none;
}

@media only screen and (max-width: 680px) {
    .aside{
        width:100%;
        float:left;
        background-repeat:no-repeat;
        text-align:center
    }

    #contactar_dsk{
        display: none;
    }

    #contactar_mob{
        display: inline-block;
    }

}