Presenting a menu within page content with associated descriptions and images
I'm not positive I entirely understand what you envision, but one possibility might be to use the Menu Trails Module to handle the submenu.

For the section of blocks you are currently toggling on and off contextually, I might consider instead authoring a block in Views that is queried off a default argument passed through the url.  This argument would be the same value from the Main Menu which is configuring the Menu Trails and could be configured to display the node teaser (or a cck/field value called Overview) for the active Main Menu Item.  

Or what you may be describing is something more like a list of submenu items as teasers,:

[Some Photo] | [Some Photo] | [Some Photo] 
[Menu Title] | [Menu Title] | [Menu Title] 
 [Overview]  |  [Overview]  |  [Overview]


In that case, I'd still use Menus to organize the content hierarchy, but don't check expand in the menu options.  Instead, use the method described above to construct a block in Views that displays all the submenu items. You won't need Menu Trails for this type of display.