Different BG image for landing page/all other pages

Using atahualpa 3.4.9

I found forum postings from Flynn about how to create a landing page template. I have the rough structure up @: http://www.reformationcomic.com/

...and yes, I know; it's non-functional. I need to add a couple form elements and a link to allow entry.

The rest of the site is @: http://www.reformationcomic.com/archives/26

I have a fully scalable bg image inserted right underneath the body with an HTML insert. I would like to have different bg images for the landing page, and the rest of the pages. I have read forums about using body.post-id to use different backgrounds for various parts (header, etc) of a page, but since my bg is an html <img> tag for the entire page instead of a traditional CSS bg image, I don't know if this will work. (Although I might be, and probably am, wrong)

Can I use CSS with the existing Atahualpa structure to do what I want? Or perhaps add $var = x on my landing page template, and $var = y on all other pages, and put...

<?php if ($var == x) {<img src == landing page image>

} else {
<img src == other pages image>
}

?>

...within the body top insert section?

Any help is appreciated, and thanks in advance.

T.