Hi fabioami
Firstly, you will need to add suffix 'frontpage' to the home menu item of your site via template option: https://prnt.sc/riv8x3
The background is declared via css:
.frontpage #gkHeader {
background: url('../images/header_bg.jpg') no-repeat center top;
background-attachment: fixed;
}
so you can change the image by replacing above URL or replace the current image: /templates/gk_steakhouse/images/header_bg.jpg by your own one.