How-to highlight parent menu for attachment in Twenty Twelve?

Hello, I am posting here hoping that someone can give me some tips.

I have a page with a gallery attached to it. I am using the basic WordPress gallery. When I am on the page the appropriate menu item is highlighted. When I open an image (image.php is used) the highlight of the parent menu disappears.

When I am on the thumbnails page, the menu has these classes:
class="page_item current_page_item menu-item menu-item-type-post_type menu-item-object-page current-menu-item page-item-11 menu-item-15"

When I open an image it changes to:
class="menu-item menu-item-type-post_type menu-item-object-page menu-item-15"

How can I make sure that the menu of the parent stays highlighted when I am on a child page? Can I achieve this with CSS, or do I need to add some new functionality to functions.php to get this to work. Any tips for this?

I am using a child theme based on Twenty Twelve, you can see here what I mean: http://dev.jeroenakershoek.nl/testbericht/

Thanks,
Jeroen