check for exceptions at server log.
There is an exception in the Tomcat log, java.lang.NoClassDefFoundError: Could not initialize class sun.awt.X11GraphicsEnvironment. I did a google search on this and found that -Djava.awt.headless=true has to be added to CATALINA_OPTS when the server is started. Once this was done, all RichFaces icons were displayed correctly. Thanks for the tip Ilya.