[Zope] All the pretty Icons have gone

As a new learner of Zope & Plone (Plone 2.5). I am slowly working through ZopeBook-2.6. Most going well but suddenly all screen icons have disappeared!. All I see is the alternative text. * Probably the last thing I was playing with is on page 110 of ZopeBook-2.6.pdf Where I viewed directly "http://localhost:8080/p_/ZopeButton" - which still works • And did some experimenting with "standard_html_header" • It was about this stage that icons began to disappear • I have done some undos • I have restarted zope • I have restarted computer • I have tried an zope.org archive search without success Everything else seems to work OK Any help appreciated. George Wright