How can I add a background image in header?

Hello Joaquin ! 
To add a background image in header of the DW Focus, you can add the following code to the style.css file:

 .site-header #header {
    background: url('FILE URL');
    height: 80px;
}

Notice: Please add the image to the Media then copy the FILE URL and paste in the line code:  background: url(‘FILE URL‘);
Hope this helps !