how can I put a banner?

on the main page, there is a big link that says "click here for spacehey html, the ultimate guide" - give that a click and browse the tons of codes on the website. the one you are looking for is under the "image codes" section. here it is:

<style>
main:before {
width: 100%;
height: 300px;
display: block;
content: "";
background-image: url('URL');
background-position: center center;
background-size: cover;
}
@media only screen and (max-width: 600px) {
main:before{
height: 200px;
}
}
</style>