I had this exact same problem. Solved it by moving the image(s) inside the resource folder and then changed the path relative to the css/sass file.
File structure
resources
├── css
├── img
├── helicopter.jpg
├── app.css
Inside app.css/sass file
.slider{ height: 70.8rem; background-image: url('../img/helicopter.jpg'); }
From the docs (found here)
Any assets referenced via a relative path will be re-written, versioned, and bundled by Vite.