Wordpress Image - Custom Header

Question

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

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

img/background.jpg

And it didn’t work. At first, I added the image to the library. Retrieved the entire address as if it were uploaded, so yes I was able to add the image through a specific path.

However, how do I customize the image? Because I intend to make the site responsive, and I need to access both the image, when the div that skirts 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;  

?>

css style.

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.

I mean, the image appears, but I can’t customize it.