Hi André,
attached you will find my comments:
>> Some movie images do not display depending on the menu item dimentions (set in the theme)
a) Please check the paths of the images.
//Add items to main_menu
AddMovie("Matrix","./movies_covers/matrix_200x300.png"

;
AddMovie("Terminator 2","./movies_covers/t2_cover_200x300.png"

;
AddMovie("Lord of the Rings", "./movies_covers/lord_of_the_rings_cover_200x300.png"

;
AddMovie("Titanic", "./movies_covers/titanic_cover_200x300.png"

;
AddMovie("Australia", "./movies_covers/australia_cover_200x300.png"

;
!> AddMovie("Quantum of Solacer", "quantum_of_solacer_cover_200x300.png"
;
!> AddMovie("Sin City", "sin_city_cover_200x300.png"
;
!> AddMovie("Superman Returns", "superman_returns_cover_200x300.png"
;
!> AddMovie("X-Men Origins Wolverine", "x_men_origins_wolverine_cover_200x300.png"
;
You forgot to set the subfolder (movies_covers) in some cases
b) You have to use a circumvention. Set the dim_left and dim_right attribute of the menu to "0". We will solve this problem ....
>> Menu should be centered at the bottom of the window
The menu is the first item of the window, so it is positioned at x=0 and y=0 automatically. Please use a gap Widget to move the menu to the bottom position. The alignment = "bottom_center" attribute mean, that the position of the menu elements is bottom_centered.
>> Selected item should be always in the middle
You can use the fixed_pos="4" attribute of the menu widget to set the highlighted element.
Happy new year!
Guido