How do I disable images in Safari 6.0? In Safari 6.0 the 'Appearance' tab from 'Preferences' was omitted.
How can I now switch “Display images when the page opens” on/off ?
Open Safari and go to Preferences, then click on the Advanced tab.  There's a checkbox at the bottom titled "Show Develop menu in menu bar".  Check that then close the Preferences window.  

You'll notice that in your menu bar between Bookmarks and Window you have a menu called Develop.  Near the bottom of that menu list is Disable Images — click it.  Now your browsing experience is image free!