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if youre talking about what I have, this was what i did

<style>
main:before {
    width: 100%;
    height: 150px;
    display: block;
    content: "";
    background-image: url('https://i.pinimg.com/1200x/b5/9d/b0/b59db0a5b2331a2dfb33ae5148481ccc.jpg?width=984&height=655');
    background-position: center center;
    background-size: cover;
    margin: 10px 0;
}
@media only screen and (max-width: 600px) {
    main:before{
        height: 200px;
    }
}
</style>