The Special menu lets you preview images, music, movies, and PDF and text files.
With previews you can also view file information, copy and move files, and open them with specific applications. Finally, previews allow you to copy images to the Clipboard, display pictures and PDF files as slideshows, and view the metadata associated with pictures, songs, movies, and PDF files.
Open the Application Wizard pane in System Preferences and click Menus.
Click the Customize Menus button. In the dialog that appears, click Special.
Make sure the "Show previews" checkbox is selected and click the Customize Previews button.