A good way to add a different background image for each page?

I’m working an a Thematic child theme wherein I need to have a unique image decorating each of the site’s main pages, from a pre-defined set of permanent images. It would also be ideal to have one of these same images randomly selected to appear on all other pages.

Using a sidebar widget to load the images seems like a good option, but I don’t know how to create my own widgets yet, and haven’t had any luck finding a plugin which provides the simple functionality I’m looking for. I also don’t want to clutter up the admin panel with plugins and options that the client shouldn’t be fiddling with.

I think it will probably be most ideal to put something into my functions.php which adds some generated markup to a Thematic hook based on the page ID, but I’m not quite sure what I’m doing, so I thought I should ask to see if someone already has a great solution.

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