Amazing how difficult it is to find out how to refer to images in a CSS file in JSF 2 using libraries. But finally I found it out with the help of a colleague.

Imagine you have an image background.png in a resource library called images. When you want to use this image as background in CSS you just write

.myclass {
    background-image: url(#{resource['images:background.png']});
}

Not so difficult, isn’t it?