Enter navigation_pic_off for handle and Navigation Picture Off for name.
Create another attribute with navigation_pic_on for handle and Navigation Picture On for the name.
If you intend to use this for all navigation items, you might want to assign the new attributes to the page types by default. Go to Page Types and click on Edit for each page type. Select the two new attributes and click on Update Page Type.
By following the steps in the time for action, you created two attributes which allowed you to assign two pictures to every page. Attributes in concrete5 are very flexible—you can create and connect them to pages, users, and files in case you have to manage object-specific data.
Attributes can be helpful with a variety of different problems; make sure you know how to use them.