In external style sheets of my current JSF project, there are hard-coded links to external resources like
.someId { background-image:url(/context/resources/i
@Bozho's answer pretty much covers your options.
Another possibility is to continue having a static style sheet, and filling in the dynamic parts in the document's head where you have access to your expressions:
in the PHP world, this is the best practice because it saves an expensive PHP process from being created for the style sheet call. I don't know how things are in the JSP world but I assume it's similar.