@media only screen and (max-width:768px) {
    .demo-gallery > ul > li {
        float: right;
        margin-bottom: 15px;
        width: 25%
    }
}

@media only screen and (max-width:420px) {
    .demo-gallery > ul > li {
        float: right;
        margin-bottom: 5px;
        width: 49%;
        height: 90px
    }
}