How to add a background image wallpaper to a drupal site from within the drupal admin? How can I add a background image wallpaper to a drupal site from within the drupal admin i.e. not editing themes or PHP?

I'm not adverse to PHP or themes but adding a background image wallpaper is a generic non-theme specific feature so should require editing a specific theme. 

This has to be one of the simplest/fundamental/popular things one wants to do with their site. It ought to be obvious in Drupal -and- possible within the Drupal admin, not via editing themes / PHP code.

Is there a way to do it? 

I found some promising modules (actively depveloped for Drupal 7) but am a little unsure how to use them:


http://drupal.org/project/bg_image (Instructions say: "provide a css selector and color and repeat attributes in the 'Default CSS Settings' section." - but how do I know what CSS selector to use? Yes I know CSS but in the context of Drupal, is there more than one I need to consider and why? Perhaps an example would help here)
http://drupal.org/project/dynamic_background (This has more advanced settings for specific entities in drupal: User, Blog, Panels, Node, Views (beta), Context) - OK fine. BUT WHICH one do I use to simply put up a background image for the pages of my Drupal site?!
For simple Drupal themes we use Artisteer. You can change most properties of a theme with just a mouse click. Please note that the tool is not free (and I'm just a happy user).