It's not much different than how you are refering to the stylesheet stored in static resources.
<apex:stylesheet value="{!URLFOR($Resource.myStyles, 'styles.css')}"/>
You have to go by the same principle to add the reference to static resource in tour css. Catch point is that your style declaration needs to be on the VF page so that it gets rendered by the force.com server. Assuming your images forlder is in the myStyles zip file as static resource:
<style type="text/css">body { background-image: url("{!URLFOR($Resource.myStyles, 'images/back.jpeg')}") }