The problem with grouping pages under a Navigation Link in the Kosel theme is that, when the link is clicked in the menu, the link's URL is opened (rather than the menu expanding to show the second tier pages).
If your 'paintings', 'sculpture', 'water' and 'ferries' pages are gallery pages, then I would recommend using Gallery Index Pages (rather than Navigation Links) for your 'Art' and 'Photography' top tier pages.
Alternatively, if you like, you could change your theme to Boma ('Site -> Change Theme') and select the 'Show all subnav' checkbox ('Site -> Customize Theme -> Navigation').
This will always display all links to all pages so links to your second tier pages (grouped under Navigation Link pages) will always be visible. (The ability to display all links at all times is available only in the Boma theme where the side menu is better suited to displaying a larger number of menu items at once.)