Here's an example answer from me for my question (I don't necessarily expect to accept my own answer, I'd rather a non code-editing in-drupal-admin solution)

Using the minimal theme / acq_minimal, located in my installation at path

    ../sites/default/themes/mythemes/acq_minimal

There is a file called 

    palette.css

In this file there is CSS for the body html tag. Edit this to add a background image:

    body {
      background-color: #000000; /* new background colour */
      background-image: url('wallpaper.jpg'); /* inserted line for background image */
      background-repeat:no-repeat; /* inserted line to not repeat */
      color: #333333;
    }

It's a shame that there isn't a standardised way to do this within Drupal web-based admin without having to resort to editing the theme code (editing the code (via FTP and a code editor) breaks the wonderful user permissions system drupal has within its admin. Fewer people (or even just one) person may be allowed FTP access to the docroot of the drupal installation. This creates a bottleneck and complicates the administration of user rights.

**Update**

Ideally I should do this as a sub theme based off of this theme (so that should the orignal theme be updated, those updates won't clash with/overwrite my change during an upgrade) - need to work out how to do this.

However this does not quite answer my question about doing it within Drupal admin. But it still gets the job done. My suggested initial solutions are also worth exploring:

  - 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?!