.custom-image-grid li {
    max-width: calc(50% - 10px) !important;
    margin: 10px !important;
}

.custom-image-grid li:nth-child(2n+1) {
    margin-left: 0 !important;
}

.custom-image-grid li:nth-child(2n) {
    margin-right: 0 !important;
}