The problem is with the path your images.
If you open your CSSMenuWriter/cssmw/menu.css file, you could see that your images are referenced to use file system:
background-image: url("file:///Macintosh HD/Applications/MAMP/htdocs/.../www/CSSMenuWriter/cssmw_images/bg.jpg");
please change it to use document relative:
I'm not sure why the css was generated using absolute file path, but you might want to check your side definition to ensure it is set up correctly.