Use this code to the Custom CSS box

div.header-menu-bg.theme-bg--primary {
    background-color: transparent !important;
    background-image: url(https://cdn.pixabay.com/photo/2023/08/19/13/42/water-8200502_1280.jpg) !important;
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center center;
}

Result: