.grid {
    opacity:0;
}

.grid-sizer,
.grid-item { width: 50%; }
.grid-item img { width:100%;
    display: block;
    height:auto;
}
* { box-sizing: border-box; }

@media all and (min-width:600px) {
    .grid-sizer,
    .grid-item { width: 33.3%; }
}

@media all and (min-width:1020px) {
    .grid-sizer,
    .grid-item { width: 25%; }
}

@media all and (min-width:1600px) {
    .grid-sizer,
    .grid-item { width: 20%; }
}