Wordpress Image - Custom Header

I'm creating a theme from scratch in Wordpress, and I would like to make a background with a custom image in the header. I'm trying to do it on the same front-page.php.

First, I tried to access the image through the path:

img / background.jpg

And it did not work. At first I added the image to the library. It retrieved the entire address as if it were up, so I was able to add the image via a specific path.

However, how do I customize the image? I want to make the site responsive, and I need to access both the image and the div that surrounds it.

The code:

front-page.php

<div class="header-image">
    <img src="enderecoDaImagem/background.jpg" alt="Roma" />
</div><!-- /header-image -->


<?php if (have_posts()) : 

    while (have_posts()) : the_post();

        the_content();


        endwhile; 

        else : 

            echo '<p> Não tem Posts </p>';

        endif;  

?>

style.css

img{
    text-align: center;
    max-width: 100%; /*Isso faz com que a imagem não saia de dentro do wrapper.*/
    height: auto;
}

.header-image{
    border: 3px solid red;
    max-width: 1000px;
}

Not even putting a border in the class it is applied.

That is, the image appears, but I can not customize it.