.image-banner__media-block {
    flex: 0 0 50%;
}

.image-banner__media-block:only-child {
    flex: 0 0 100%;
}

@media screen and (max-width: 768px) {    
    .only-first .image-banner__media-block {
        flex: 0 0 100%;
    }

    .only-first .image-banner__media-block:nth-child(2) {
        display: none;
    }
}