allow blog owner to customize theme

Hi, i'm working on a project ( i decided to go with creating a template set ), in which one of the features will be allowing the owner of a blog/site to upload his/her own background & header images to create a custom look/feel, in conjunction with a list of predefined themes (mainly combos of premade background/header images)

can anyone point me in the right direction as to how would i go about creating a page/module in the template set which would allow them to do this (restricted only to a blog owner / admin). how could i sort out where the uploads go and check if they're there, or i suppose i could just store a theme variable when they upload, but where do i store/retrieve that specific information? I'm sort of new to movable type and the list of template tags is daunting. Has anyone had to build this sort of customizability into their site? Is this something that should go on the backend instead of the actual blog/site?

i'm using version 4.3.2 enterprise with Community Pack 1.64, Enterprise Pack 1.3, Professional Pack 1.3

thx,

n