Just to clarify you want the something like the following?
when you hover over Garden photo's sub menus appear for say Gallery 1 and Gallery 2?
To achieve this you can use the menus tab found at
Dashboard> Appearance > Menus
Simple use the large block on the left to name a menu. Add the pages you want to display as the menu using the "Pages" "Cusotm Links" and "Categories" on the right. Once you have added the pages to your new menu you can drag/drop them into the order you would like them to appear on the menu. You can also indent which will make it a Child of the page directly above.
so you should have something like this at the end
- Gallery 1
- Gallery 2
Once completed click save. Then the last thing you need to do is under "Theme Locations" On the left you need to select whatever you called your menu under 'Primary Menu' and click save.
Hope that is clear and makes sense.