You can use this code to Website Tools > Custom CSS to change burger overlay background color to custom image.
Replace Pixabay with your desired image url.
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
