Using CSS files is the recommended way for incorporating the chrome in a SharePoint WCM solution. Among the various definitions of the layout pieces there are also images' references. Each such reference can be defined using an absolute of relative URL. As long as you use images in the same directory as the CSS file there is no problem at all: you can manage all the paths easily by simply using one of the URL kinds. Unfortunately a problem occurs when you would like to use images from other directories for example the PublishingImages folder. They should be referenced via relative URLs. Using absolute URLs results in problems when a Site Collection doesn't reside in the root or /sites directory. Also sharing a CSS file among different Site Collections isn't possible while using PublishingImages folder: there is no simple way to make the images' paths be defined dynamically based on the Site Collection calling the CSS file.
In his recent post Robert J Wheeler has presented his way of dealing with building CAML queries dynamically. Here at Imtech ICT Business Solutions we use a slightly different approach.